Here are my tips:
Go in with a plan: Know what you need and don't need. It can get overwhelming so having a good idea of what you need will help the process.
Study up on current trends: Look in magazines and fashion blogs for inspiration on what is in style for the season. I think Forever 21 is a great place to buy trendy pieces. It won't matter to you if they are out of style next season because you didn't spend much anyway.
Look for special pieces: I mean look for things that you may have seen in another more expensive store. Usually Forever 21 will have "look alikes" at cheaper prices.
Edit: You don't need to come out with 6-7 new pieces just because they are cheap. I am SO guilty of this. Go in, look around and buy a few things you like. You don't want to dress head to toe Forever 21, mix it with your more quality pieces.
Give yourself plenty of time: The store can be a time suck so I focus on the non sale items. It takes all I have just to look through the regular priced items so I usually don't bother with the sale section, which is unusual for me but keeps me sane.
This is an example of a good trip to Forever 21: You can't have enough striped shirts. The black dress you can wear now and add boots and a sweater when its cold. The patterned dress is fun and easy and the mixed faux leather jacket is something that is very "in" right now. I actually have that necklace and love it and tassels are very on trend this season. Go crazy in the accessory department.