There's an old thread on the old Megatokyo forums [http://forums.megatokyo.com/index.php?showtopic=1715168] for a Character Design 101 class from a professional artist, who's one of the mods there. The forum is a lot quieter and less lively, but that character design class is about as close to a professional design tutorial you can get on the cold, distant wastes of the internet. Bounce through those threads, and you'll find a wealth of knowledge that an art hobbyist like myself would never be able to give you.
If you want the quick TL;DR from me, some quick tips:
1. Outline - One of the best uses of a character is understanding that the design should be comprehensible even in its most abstract form. If all of your characters were just silhouettes, they should still be mostly identifiable compared to their peers. This means deciding on an outline and major features that are clear, visible, and comprehensible at a glance. This will help make all future aspects of the design more directed as well.
2. Major Theme - Figure out what works for this character. For example: Tali (Mass Effect) has a clear hood and helmet structure, one that persisted throughout the trilogy. Does this character use big weapons? Heavier, stockier build? Faster character? Figure out what major themes you want your character to run on, then build from that. Heavier characters should have bulkier, heavier armor to emphasize their weight. Light characters have open apparel for air flow to the skin (lots of sweat), but also to emphasize the drifting movement in action (lots of flowing ribbons and fabrics, for example).
3. Simple Design - Simply your design as much as you're able. Keep the parts of it that should be iconic front and center, and try not to add too much noise otherwise. This will simplify the process of keeping character on-model, as others in this thread have suggested, and will also help keep the characters strengths in mind when designing.
Hopefully this points you in the right direction. There's a lot to unpack in character design, so you've certainly set a Herculean task for yourself.