When I shop at Forever 21 I am looking for two thing: #1 Super trendy pieces and #2 Super cheap basics. I am not looking for regular rotation pieces, but my main goal is to look for pieces that I don't mind wearing for just a season.
So I tend to really stay towards the front of the store, that is where the new pieces are, and therefore the latest trends. All the pieces I purchased the other day (besides the green dress, which was the only one like it in the store) were from the front area. Hot trends like florals, military inspired and striped pieces were heavy in the front area. I was hoping for more strong shoulder pieces (because I think this trend is SO flattering and hip) but there wasn't all that much, but the cardigan I grabbed is amazing!!! I find that F21's jackets, dresses, skirts and cardigans are my favorite. And I try not to buy pieces that have a lot of stuff on them, for instance, sequins. I have had bad luck with pieces falling apart after a wear or too so I don't buy a whole lot of that. I also grab a ton of stuff try on when shopping there. I normally HATE trying on clothes but when I shop at Forever 21 I plan for it. I went to the dressing room three times, each time with pieces I thought were awesome, but the fit was not.
After looking for the trends I want, I look for cheap prices. I picked up a sweet grey and white striped skirt for a mere $6. I am certain I will be definitely be getting more then $6 wear out of that. Forever 21 is also a good place to get dresses and tunics. They have great plain basic ones for great prices and cute ones with great details (like pockets and pleats) for steal. I was SO excited about the green dress I picked up and the sun dress has pockets, pleats and a zipper, both were about $20!!
Oh and I always hit up the Jewelry and accessories area, they have GREAT pieces for a steal.
My haul:

Although Forever 21 is a great store for trendy pieces I do feel a bit guilty shopping there (ethics, environment, etc) so I tend to limit my shopping there but it sure is a wonderful store for great styles for less.