You're already doing well at what many people find the hardest part, which is walking up to strangers, talking them into posing for a few shots. In addition to helping you get a good shot today, this makes friendships and connections that help at the next convention.
In terms of shooting basics, many have already been brought up.
1) Keep an eye on what else is creeping into your shots -- people, a cluttered background, etc. You can usually find a better angle that improves these things, or ask your subjects to move a bit. When you are planning a more substantial shoot, scout out other areas you can use in advance. Ideally, do it at the time of day you'll be shooting, so you know where the sun will be/etc.
2) It's tricky to shoot in harsh sunlight, and in many of these shots you are shooting against the light. That is, the sun is behind your subjects, leaving the side you are shooting in shadow.
3) Get more creative on distance and angles. You don't need to capture everyone head to toe, unless the costume/pose warrants it. Get in close - it's the eyes/expression and filling the frame with your subject that often sell a shot.
4) Like most of us, you got a wide range zoom lens as a first lens. It's tempting, because it seems to be able to do everything. But in fact, a zoom like that makes a lot of tradeoffs in image quality and aperture. It is tough to take quality pics with them. Try to borrow a cheap prime lens in the 35-50mm range. One advantage of a prime lens is that it generally offers a much wider aperture, which helps to blur the background and isolate your subject.
5) You don't need cosplayers to practice! Bring the camera along when you're out with friends or family, at other kinds of events, etc. Most will humor you. Try to recreate shots you've seen and liked.