Playing the Guitar

I've been practicing guitar for a while now, trying to teach myself, but I'm still having problems with my up strums. It may just be a matter of experimenting and practicing more but I guess it can't hurt to ask around for some help. The thing is, I often hit the strings either too hard, not at all, or just get a messy strum when I play. Could it be that I'm trying to do too much of the movement with my wrist? How much should I rotate my wrist between up and down strums?

I try to keep my pick as close to a 90degree angle with the body of the guitar as possible when playing, and only tilt it a little (point it a little upwards on down strums and down on up strums). Should I tilt it more?

Any help is much appreciated. I know that experience will make me better but I'm trying my best not to develop any bad habits so I really am hoping for some advice.