Practice. It'll even out eventually. I don't know whether everybody is uneven to start with, but most of us are, for sure. The more comfortable you get with the process, the more you trust yourself that it really does work, the more even your product will be.
If you're using washable yarn, a trip through the laundry will shake out a lot of unevenness. Of course, you could just do felted proijects for a while :D I LOVE felting. It hides all my mistakes!