Keep a running list and know what you like.
I typically keep a mental list of things I'd like to look for at the thrift shop. I've been hunting for a denim jacket for forever, but I've yet to find one I like, so that stays on the list. It's a lot easier to go in with a plan than to wander around looking at every piece in the store (unless that's what you want to do). I know that I tend to go for oddball patterns, stripes, and sweaters, so I visually scan and find most things that way. I do have days where I just go through the racks, but on quicker visits I've gotten it down to an art.
Learn your fabrics.
If you're looking for clothing, learn to identify fabrics just by sight. When I was sourcing for vintage a lot, I'd learned what polyester and linen looked like just by walking by and seeing them. Granted, there are exceptions as rayon can look really similar to cotton, but with a general knowledge of knits and such, you can easily find things you're looking for.
Have a buddy.
I'm fortunate that my two best friends are thrifters, too! Jess and I meet up for thrifting trips and always find some amazing stuff. It really helps to have someone with you for when you pick up that odd piece and want an opinion on it, or you need someone to talk you out of an absurd wall hanging.
Don't always go by the tag size.
Clothing sizes vary wildly and so do folks' laundry habits, so don't always go by what the tag says. Don't be afraid to try things on or hold them up to your body. Look through all of the size racks and remember, especially if you're looking for vintage, that a size 14 could really mean a modern size 4.
Follow thrift store etiquette.
Don't take things from others' carts, replace items where you got them or on the fitting room return rack, don't bombard the workers that bring new carts of merchandise out, don't eat in the store and then touch merchandise with your greasy hands, and be polite.
If you have any more tips, leave them in the comments! :)