Initiative has been a mechanic in the D&D game since the earliest days, evolving from heroes/adversaries pattern to "which segment does my long sword attack with a speed factor of 5 happen" to the current standard of "higher is always better".  Keeping track of initiative, as a result, has changed a lot as well.  I created a "rolladex" using standard playing cards with Ace-10 to track segments during the era of segmented combat (1E-2E) to writing names next to numbers (30+ to - 4) to briefly having PC names on cards on my DM screen in initiative order.  These days I use 3"x5" cards with PCs' vital stats written down on them and a corner of the card devoted to their initiative number.  The bad guy initiative card has their hit points on it for tracking damage.  What is your go-to, fellow DM/GMs?

For the longest time a whiteboard where player or character name was listed with whatever they roll + initiative mod. That quickly became difficult to manage with a group of 10 players. Since then people roll (monsters included), DM will count down from 25 and when a player's roll+mod is said, they indicate themselves and receive a folded note card with a number on it. 

In combat the DM asks for number one, number two, and so forth. It's been much more organized. 

Old style pen and paper. Although, I write also the number of initiative. There are some situation in which you have to recall the exact value of each initiative roll.

I'm a new DM and I wanted to be able to use Tech to help me manage my game. I chose to use the APP Fifth Edition DM Tools on my Tablet and I have my Players Characters within it and my encounters entered in. I ask for their Initiatives at the beginning of an encounter and the App helps me keep track of whose turn it is and current Hit Points of the Creatures and the players. So far has been working out quite nicely for me. But in the Group I'm a player in it's Whiteboard all the way.

I use sticky notes for covering my maps (removing them as players explore) and use the removed sticky notes for tracking initiative/hp of creatures once combat starts. New combat, new note.

Quote from Stormknight >>

The game encourages all similar creatures to act on the same initiative number, so reusable cards like these made sense.

I then have one of my players look after placing them in the correct order when initiative is rolled for the start of a fight scene.

I use one of these (well, the older model, but still pretty close) for initiative tracking in D&D

Since I play on Roll20, it has a built in Turn Tracker. In the past before Roll20 I used to just track using post-its that I would stick on my DM folder. Easy to move around when people would delay and such.

First,  I use the Pathfinder Tracker also, but most methods work.

That said, the most important thing I have found, after years of playing, is let the players keep the initiative tracker.  

1)  One less thing for me to do. 

2)  amazing how much faster combat runs when the players have the initiative tracker in front of them and start thinking about what they will do BEFORE you ask them three times, say "yes its your turn", etc.  keep them involved and their quality of play and enjoyment of playing will increase.  

Oh, yeah - I totally have one of my players keeping track of our initiative tracker.

Since handing it over to him for tracking, I'm the only one at the that ever has to ask "Who's next?", and that's even only occasionally because he's distracted with something else rather than telling me which creatures are up like usual. Everyone else can see the tracker laying out on the table, and the player doing the tracking is the one most into the tactical aspects of the game so he's usually right on top of telling everyone their turn is now or coming up next.

I have players handle as much of the bits of running/managing a campaign as I can. It's key to my "lazy" DMing style (scare quotes because I'm far from lazy, I've just minimized the amount of things I have to be doing so that I can focus more deeply on the few things that I am responsible for - using my brain-power for depth of PCs and worldbuilding, rather than remembering to keep notes or track things).

I have a bunch of old tiles. My players all got to decorate their own and I have plain white ones for everything else. They all have a little space so I can write the initiative roll with whiteboards markers.

Quote from AaronOfBarbaria >>

I use one of these (well, the older model, but still pretty close) for initiative tracking in D&D

My DM in a game is a bit of an artist.

He gave everyone small cards (~2"x6").  You draw a pic of your character with his/her name on it.

On the back you write your Character Name, Player Name, AC, passive: Insight, Investigation, Perception.

He then uses the GM screen more as a "board" and hangs the cards in the order N/PCs go.  It makes it super easy to track where we are, plus everyone has a little card.

Plus the GM has instant access to all the basic info for each player.

For non artists, you can print a small pic for the front.

I use roll20's turn tracker for online games, and the Pathfinder Combat Pad that Aaron mentioned for in-person games. Either way, the initiative tracker is public so people know where they are in relation to each other and the enemies, lets things run more smoothly.

I use 3x5 cards, flipped into portrait mode and folded in half to form tents.  This are placed on the top of the DM screen.  The side facing the PC's has their Name and a quote.  For example, every time our bard introduces herself, she says "You may have heard of me?"  So, this quote is under her name.

On my side, I have their Name, Race, Class, AC, PP, Max HP, if they have Darkvision, and some other notes so that at a glance I have any info I might ask them for.

It works really well for our group.

There's an old program I used for my 4th edition campaign for campaign management and combat tracking. I still use it for 5th edition as well even though the bulk of the mechanics were designed with 4th edition in mind.

Masterplan

I'm hoping that DnD Beyond will be a great replacement for it!

Quote from subsistcyber >>

There's an old program I used for my 4th edition campaign for campaign management and combat tracking. I still use it for 5th edition as well even though the bulk of the mechanics were designed with 4th edition in mind.

Masterplan

I'm hoping that DnD Beyond will be a great replacement for it!

I have only just started DM'ing, and as my group are all school students I use an iPad with the 5e DM app on it. This allows me to track combat, have character sheets available, and gameplay notes accessible all in one place. In fact the only real reason I use a physical DM screen is to block the view of the iPad screen, and hide any maps/handouts!

I tried lots of things over the years, but a deck of cards is the most effective and efficient I have discovered.  Especially since I tend to have large parties and large battles.  Here is a writeup.