I recently started developing severe pain in my wrists after slaying over my drawings :/ Do you personally have any useful prevention tips?

Yes, stop drawing. Stop typing. And stop for at least a week, preferably a month.

Sounds harsh, but it’s the only thing that will heal tendonitis, and even then it will keep coming back if you keep injuring yourself to the point of severe pain. I learned this the hard way, now I have to wear these:

Take frequent breaks and never let yourself get to the point where you are feeling pain and try pushing through it. That pain is your body telling you YOU ARE DESTROYING ME!! STOP IT!!!

I'm thinking of investing in a tablet. I've been drawing for a while now with just my mouse (which is a pain and takes much more patience than I'm capable of) but is a tablet really somewhat of a necessity for drawing? Does it really make a world of difference?

Yes, it really, really does.  Drawing with a mouse is not only really bad on your wrist (it’s extremely unergonomic) but it also really limits your capability.  You can’t control strokes well, and there’s no sensitivity to pressure like you get with tablets.

I’ve never known a professional digital illustrator who drew entirely with a mouse.  If you were super dedicated, I suppose you could painstakingly point and click vector art to have smooth strokes and tapering, but having done that and then tried it again with a tablet, I can honestly say it’s not worth the trouble (or the time.)

I don’t even have a mouse anymore. I use my tablet for everything, even internet browsing.  That’s how I got so accustomed to it.  Once you get a tablet, you’ll never want to touch a mouse again. =P

Also, I can’t vouch for this personally, having never used one, but I keep hearing really good things about this new tablet by Monoprice.

I currently use a Wacom Intuos 4, but if I were to buy a new tablet right now, I’d go for the Monoprice. It’s cheaper, and apparently just as good (if not better.)