In my games, I sometimes use poster maps given in pre-made campaigns but they give away the rest of the dungeon. How should I work around this? What is the best alternative? Please help!

Fold the map up or tape plain sheets of paper around the part you don't want the PCs to see.  

There aren't great options, but you can use sheets of paper to cover the areas you don't want them to see.

I like fabric to cover my dungeons. It’s easier to fold it and bend it around as the party enters each room. 

Last night I used sheets of paper towel in a pinch to cover what characters couldn't see. Quick, cheap, and good enough for me.