Long-time cosplayers will tell you to know your body type. Know what looks good on you, and know what kind of costumes you can (and cannot) pull off.
I've never cosplayed before, and I will not try to be so presumptuous as to tell you what you can and cannot pull off. I will, however, tell you why I chose the costumes I did for my sister and myself.
I, for one, am a little on the petite side. Alright, A LOT on the petite side. I fit into size 0 jeans and have a hard time buying things off the rack that fit. #1 reason for the need for custom-made costumes.
I may be too skinny for Lee, but I sure as hell am too skinny for most of the fine lot of characters Naruto runs amok with. But Lee is my favorite. That's why I chose him. I am a firm believer that if you don't like a project enough, you probably won't have the drive to complete it to the best of your ability. You may lose interest, you may slack off, or you may just look lackluster when you're done.
I want to have a shiny happy smile at the end. Therefore, I am going as Lee.
At past cons, I've proven bright, alert, energetic, happy, driven, etc. I look more youthful than my real age implies. I'm of a lanky build. I smile a lot and get excited easily. I have few real social skills, but am polite. ...Do you see where this is going?
Mainly it is a personality thing. For the purpose of photo-shoots, I can be Rock Lee.
Remember, cosplay is portmanteau of "costume" and "play," implying to me that you are dressed as (and playing!) that character. This isn't Halloween. You don't just strut around in your green spandex and say "I'm dressed as Rock Lee." You live as Lee. Yosh! about. Smile. Be nice. Hug a Gai (and cry...bring your own sunset). Throw kisses at a Sakura. Live it up! If you aren't being the character, get the heck out of the costume! This is acting, Live Action Roleplay, some call it. Act!
My sister is my opposite in many ways. She's shorter, a bit heavier, and far less ADHD-ish. I like to say she's Kakashi to my Gai (ignoring for this brief minute that I ship them together). She's Neji because she can reign me in, not because she can look especially like him. But she'll make it work...because she'll be Neji, not just a girl in a Neji costume!
I've seen great cosplays ruined by the fact that a person was acting out of character. It's okay to be out if character for a while, but if you're taking a picture...uh, duh! Be your character! Don't be blah, either. Even if you're Squall, give a good moody face or something. Be creative. Photos are immortalizing you!
So the point is: Love the skin you're in. I don't care if you look like Ino. Make your best attempt to look like her, and then be Ino while you're dressed as her...Do what Tim Gunn says:
Make it work!