Okay, I've read a lot of what other people have said, but here's my addition: don't be afraid of what the piece is. By this I mean, just because it's a foot doesn't mean you have to use it as a foot. Use masks as shoulder pads and torsos as legs!! My self-MOC, Norcon, uses feet for his upper arms. That's where the build originated, then I moved forward from there. Another MOC I have uses Metru-Nui torsos for his upper legs. It's a staple of my style at this point.
Additionally, don't get caught up in small details. The MOC is going to rely on its basic construction far more than it will its embellishments. Focus on the model and structure, then make it pretty. Prettier?
Don't confine your MOCs to Toas or humanoid characters!! Venture out and try building a quadroped, or a two legged creature with small arms. Make your own heads for these beasts, give them character! Make them oppose your heroes or your villains!
And of course, as trite and truth go, be sure first and foremost you're enjoying your creations and having fun building them. This is far more important than any praise on the Internet for it. If you like what you built, nothing else should matter.
Build on, kid.