I don't remember having any excessive problems drawing runes, though I use a lot of mouse gestures in the browser so may be more used to it than others.
I'd suggest first making certain you have the right idea about what's expected for any rune, what it looks like. I say this because I remember watching a playthrough of it once and the person had a lot of trouble with some runes, one in particular for ages and it was because they had the wrong idea of what it was meant to be.
Just to be sure here's what Cosum should look like, if you have mouse gestures in your browser you can even open a larger image and trace over it for practice. http://wiki.arx-libertatis.org/Cosum
Some parts of the gesture don't seem to matter, for example with Cosum if you make the little tails much longer / or the box much smaller (relatively) it should still work if I remember right. You can eventually learn the little behind the scene rules if you pay attention to when runes fail, that will help when you think you've made a mistake but it's actually still possible to continue casting a spell successfully.
I think the problem most have with drawing the runes is it's quite strict on the straight lines (be they horizontal, vertical or diagonal etc). If what's meant to be a straight line has a curve or tail it's not going to work, even a very long straight line with the tiniest of tails could turn Aam into Morte. That said you might find it easier drawing short lines resulting in smaller runes, the longer you draw a line the more chance of you curving off etc, and the size of the rune doesn't matter (within reason).
Less relevant but something I found handy, was instead of trying to always draw spells runes ontop of eachother, simply start the next rune where the last ended. It can make things quicker and sometimes easier to remember.
For example I still remember drawing Fireball like this, except of course with small breaks in the line at the right spot. http://i.imgbox.com/LG1VnXWK.png