How do I link two adventures together? I typically just have an NPC mention the place/conflict in a tavern scenario. But what are some other options, I trying to mix it up.

Well, there is always a chance that the party can find a map of mystery or some notes about a new adventure location in with the loot from the end of the last adventure. 

I’m not sure what you mean by “link”, could you clarify?  Do you want them to be parts of the same story, or do you just need a reason for the characters to start off on a new quest?

If it’s the first reason, you could use a theme.  My campaign features a theme of Law vs. Chaos, which I use to make individual “episodes” fit the main plot, which I’m still working on.  For example, this week my players took down a Kobold wizard who was trying to attract the attention of Orcus, the demon prince of undeath, by summoning as many ghouls as he could.  They will eventually find out that this is an example of chaos run rampant— they will also discover examples of law run rampant as well.  The point is, I can put together any adventure we want, and it is easy to fit it into the campaign theme.

If the latter, depending on how your world works, an adventurer’s guild or a group patron could provide access to new quests.  In my world, there are notice boards in every major city, which the PCs can consult if they need something to do.  I keep a short list of quests handy, to represent the notices on the board.  These quests are minor adventures which I have obtained or invented, and always have prepared.  I also intend to implement a group patron, who just so happens to tie in to one PCs backstory.  Tasha’s Cauldron of Everything has lots of ideas for group patrons.

Do you use the alternative "Honor" and "Sanity" attributes in your campaign?  (You should maybe consider adding them on the sly if you haven't.  Younger players are very highly motivated by the "Like"/"Don't Like", peanut gallery consensus. 

If the players are regulars, they have some loyalty to each other, right?  But belong to different classes and races.  So if you refer to the back of the 5e Dungeon Masters manual (available on this site) there's a whole section in the back with tables of suggested stock tricks beyond the old,

     "So you're having a drink in your favorite tavern when... A rabbi, a Priest, and a drunken half-Ogre [or Tiefling or other racial type not currently in your campaign, but which would make for a cool Henchman/NPC] walk into the establishment."

Players ALWAYS want something badass.  Sell the greediest on an easy kill quest to retrieve a Portable Hole from some under powered enemies.  [Drool]  Then surprise them by having something actually be IN the Portable Hole that they weren't informed of by the client.  (I call this the "Firefly" or "Serenity" gamut, after the Buffy the Vampire slayer creators hit movie.  In that example, the 'something' is a crazy teenage girl with magical (ESPer) powers... )  So if you don't have to tip toe around Moe game elements (the Japanese kind, not the Three Stooges kind), a Crazy hot, obscure racial category, foreign beauty... in suspended animation within the Portable Hole, is an interesting and universal motivation. Have her NOT speak 'common' i.e. English (ideally she speaks a foreign language you are kind of familiar with)

I played in a tourney where the GM/DM brought in a smoking hot Chinese female exchange student Player, and as tournaments are always sausage fests, everyone was exceptionally nice to her while bickering and competing to get her at THEIR particular table the next session... honestly, it was one of the smartest tricks I've ever seen a GM use, especially as she didn't really know how to play her wildly OP character, but had seen enough Kung Fu movies to come up with a lot of funny ideas [that had nothing to do with D&D, but... you know Jackie Chan Heroine cliches, right?]

You know what your own players would collectively consider 'badass' better than I.  Ideally something that benefits the entire party, but is still ultimately within the DMs authority to remove from the campaign (in the old days, EVERY DM had to contend with the "We're strong enough to 'Subdue' a DRAGON!', and a couple lucky dice rolls later, the campaign spiraled out of their control:  "We have OUR Dragon do this!  We Rent out our Dragon to do that!  We have our Dragon perch on top of the local Nobles castle as an implicit threat! &etc). 

My feeling is that 5e rules should always include "Honor" and "Sanity" attribute scores along with the six others, just to allow DMs to oblige players to role-play in order to retain their characters uniqueness or coolness factor.  The WoW/Dragon Age/Witcher video game effect, has been to minimize things like Thieves Guild Fees, established State Religious Authority, corrupt law enforcement, normal taxation.... lol. 

Give them a print out of a Tax Bill from the ruling Authority of their home base, and have it be preposterous.  Of course the bureaucrat IRS official is willing to look the other way.... IF they do X, Y, or Z!  Strip them of all their hard won gear, which has been seized in lieu of paying their tax bill... he MIGHT agree to let them have it back, but what's to stop them from simply running away?  [This is called:  Lawful Evil, 101.  Arbitrary Taxation]  Even Lawful Good Players cringe over having to defend the States Right to Tax to pay municipal expenses that obviously benefit the greater good, like sewers and water and fire brigades.  lol.  Get someone you know who pays a millage tax to give you an old receipt... SEE all the GOOD things you're party has neglected to pay it's fair share for!  Aren't you ASHAMED!  Doesn't the idiocy of some of what they want you to pay for drive you CRAZY! 

(The GM/DM should always be the most evil MFer in the room.  You've got to get ruthless.  Is a Neutral Extra Planar Character REALLY going to pay municipal taxes to a Corrupt bureaucrat!?  Well, Lawyers cost $$$.  If players balk, which I ALWAYS would... hit their Honor attribute.  "Ignorance of the Law is NO Excuse for Breaking the Law!"  "F*ck their Laws, I'm a freaking Elf from a distant Land!"  "Oh? You Elves are practically the definition of Racist!" "Racist? Humans breed like rabbits!" etc.  Priests of any alignment can't exactly refuse to pay their own Church it's 10%, or disobey their hierarchy.  Fighters have repair bills.  Mages have Guild fees, & always need spell components.  Bards... offend nobles, skip Thieves Guild fees, and run up tabs at brothels and taverns buying rounds to get material for their ballads.)

Players might be exceptional killers of monsters, brave and heroic and all that jazz.  But to rulers they're just Migrant labor.  To those rulers bureaucrats, Adventurers are an ideal source of Tax revenue, at least until they get a political patron.  And GUESS what a political patron is going to want from Adventurers?  lol.  Completely unreasonable, secret and dangerous quests to advance their noble patrons political position!  "I have paid a fortune for statuary, Dwarf forged equipment for YOU, as well as my personal guards, Rare spices for my chefs, and etc.  and it HAS ALL BEEN STOLEN.  Will you please find out what happened, and retrieve it if possible?"  How can they say, "no"?

What I mean by linking, is having it be a consecutive story. 

Quote from Cindrellic >>

What I mean by linking, is having it be a consecutive story. 

 In that case, I recommend a theme; it’s my favorite method of linking adventures.  This could be as simple as undead, or as complicated as the Blood war.  Put together a handful of adventures, imagine how they might be subverted or broken by the players, and think about how you might handle those surprises, and voila!  You have a campaign.  This is pretty open-ended, though, which can be overwhelming.

Another possible idea is to create quests tied to your PCs’ backstories.  If they have personal goals or aspirations, you could do a special story arc for each one, or maybe one big one that ties all of them together.  A variation on this idea is to have the party come up with a shared goal (e.g. collective apotheosis), and let them work towards that.  This is almost always popular with players, though it can take more work on your part to make their desires attainable.  If they want apotheosis, for example, you have to think up a way for that to happen.

And of course, there’s the classic group patron concept.  This approach is nice because you can think up whatever you want, and then hand it to your players via their patron.  It can be difficult to maintain a reasonable amount of player agency, though; if you give them a quest, and they don’t want to take it, you could be stuck.

You could also combine multiple ideas in various proportions.  More ideas means more grace for you when your players derail your stories!

You could also try having an item in the treasure at the end of the first adventure to spur them on to the next. Mysterious staff, document, or weapon could do the trick. 
Another idea would be to have a small thing in the first adventure send them to the location of the second, before continuing on with the first adventure. It could get your players wondering about a local interest and then send them that direction after the first adventure has concluded. 

I intend to use "minor" quests to link adventures. Finished the main plot in this area? There's a trader seeking protection for his trade caravan as they move onto pastures new. Or a ship seeking crewmembers to protect against sea monsters. When their new "minor" quest ends, the players are in a new place, and hopefully the plot hooks are all dangling before them.