I suspect you're already doing the thing that's the only real solution - practise! Don't think of it as wasted paper so much as more knowledge and experience for next time. Have a little think about what didn't work out quite the way you planned and whether you can identify why then keep that bit of knowledge tucked away so you can use it in future.
Like Christi, I sometimes do a quick "mock up" of someting with scrap paper if I want to test placement and so on. I usually use paper that's been through my printer but has one side still blank. A stamped impression will never come out that cleanly and I don't colour it or anything but it can be a good way to think through what you want to do.
I think the biggest thing though is to remember that it's your way of having fun - just relax and enjoy the process of making something without stressing over whether it's going to turn out "perfect". You'll probably be a lot happier with those projects than the ones you've worried over!