PLAGIARISM
Seriously though, try out designs you see on other MOCs, see how things work. Eventually you’ll be familiar enough with everything that you’ll start to synthesise your own ideas. Just remember to credit every time!
Though if you look at my particular style, more often than not I still don’t use custom limbs. They’re not the be all and end all. Just depends on the build style you have.
And as rota lorem says, proportions are key. Whenever I’m building I tend to stop and stand up and look myself over to make sure the proportions roughly work. Like how far down the leg should the fingers end, that sort of thing.
Of course, if you’re carrying a few more pounds than you should like me, it doesn’t ‘quite’ work so well. And then there’s the fact that these are basically action figures, who don’t have accurately human proportions. Check out any superhero action figure or similar and compare them to a real human being and you’ll see they’re not quite the same shape.