Okay, maybe this is cliche, but it's true. There's nobody who can play the part of you better than you can.
Once you've spent 30 minutes or more studying improv comedy, you'll meet several greats via books (or if you're lucky enough, in person), people like Del Close, Sharna Halpern, Keith Johnstone, etc. And, all of these greats will tell you what they've learned, and how to become a great improviser.
But, there's a problem. Too often people change the unique aspects of themselves and try to be somebody else. To put it another way, there's already a Tina Fey, a Mike Myers, and there was already a Chris Farley and John Belushi.
Trying to imitate the greats sort of means that you won't ever be as great. I mean, Michael Jackson's theoretically making more money after he's dead than any of his Michael Jackson imitators will make in an entire lifetime.
People want to see something original.
What this doesn't mean: Thinking, oh I can do bad improv because that's me. Blocking and talking over people because "that's just me" isn't what I'm getting at. True, you might be a natural blocker, denying other peoples' offers in real life too, but if that's the case maybe improv comedy won't come naturally--it might require a lot of work. And sidenote: the people in your real life might think you're a little controlling.
What this does mean:
It's okay to be a consumer of what you learn. Take what makes sense and incorporate it. The rest you can think about, but you don't have to make it yours. There are lots of great improvisers that seem to be going against mainstream teaching, but gosh darn, they're just so interesting to watch on stage...
It also means, know what you're good at. If you're witty and bad at characters, don't beat yourself up for being bad at character acting--it just means you're good at something and bad at something else. Now go out, and showcase your strengths. Likewise, if you're good at physical comedy, but those pesky words don't come naturally to you, well... don't beat yourself up because you're only good at physical comedy.
Last time I checked Jim Carrey was doing all right. Imagine if he would have calmed himself down because an acting teacher told him he was overacting and wasn't allowing the truth in the natural situation to breathe...
Being yourself is risky because nobody's ever done it before, but it's a good risk to take. Worse case scenario, you fail, and people don't buy what you're selling. But, that might be better than if you succeed in pretending to be something you're not.