That said, here are some brief tips for how different kinds of creators can get conditioned:
Artists: Draw stuff you don't normally draw, and especially stuff you particularly don't like to draw. For example, if you hate drawing backgrounds, then guess what? Your homework assignment's to do a landscape. If you have trouble with feet, then your sketchbook better get some drawings of one of your characters barefoot. Please don't get cute and try to come up with clever ways to hide your weak spots, because sooner or later it'll be really obvious that you just can't draw them.
Writers: Write all the time, especially when you're feeling lazy and/or tired and don't want to write anything. It doesn't really even matter what you're writing about as long as you're conditioning yourself to keep putting words on the screen (or keep your pen moving, if you're old-fashioned like that). If you keep doing this, I guarantee you'll write better, faster, and with more confidence. Also, get your stuff peer-reviewed as much as you can, even if it just means exchanging critiques with other writers. If you feel nervous about having your work criticized, then it means you need to condition yourself to handle negativity better.
Reviewers: Get your word counts up. I've read a lot of webcomic reviews that are in the 150-to-350-word range, and it's just extremely difficult to write in such a compact way while also being descriptive and interesting. For a comparison, Wisp of Yes Homo is averaging around 1,000 words per review, El Santo of The Webcomic Overlook's "Ridiculously Long Webcomic Reviews" were averaging more than 1,300 words when he was posting, and my recent reviews are averaging almost 1,500 words. It might seem like an intimidating amount of writing to do, but you can condition yourself by increasing the lengths of your reviews by 25 or 50 words each time until you eventually get there.
Having the patience to slowly get better at your craft is more important than having talent. It's true that amateurs don't have as much time to create as the pros do, but amateurs can always work on being more professional.