1Start with the bed. Buy a duvet, colorful throw pillows!! A small and cute rug beside it! Also, you can find tutorials on how to make a duvet, and throw pillows are relatively cheap to make yourself, and therefore easier to coordinate with your room!
#2You make get some double stick tape *test it first* and put around the top of your wall and get some fabric and make mock curtains.
#3Make some cute corkboards! Spray paint them a base coat, then paint flowers and whatnot to make them pretty and ready for whatever pictures and reminders you may pin to them.
#4Wall stickers, made to be removed, I know there is a good site somewhere...but you will have to find it on your own, I believe the first post had some good ones.
#5If your walls are white, do not worry about small holes, that is what spackle is for, also good if someone is a clumsy and makes a small hole in the wall *you may need to use some mesh made specifically for this to put on then spackle over. good as new!*
#6Asian hanging paper lanterns! Ikea sells them for a really nice price! Basically it has the cord with the light socket, and they sell the paper spheres to go over. Get a couple you like and if they are plain, why don't you paint them? This also saves what small space you have, having a hanging lamp.
#7Go vertical, more space that way. Hang curtains starting from the ceiling to the floor, giving the illusion of a bigger window.
#8Get cute wallhooks. Try UrbanOutfitters.com This will help save space and provide cute organization.
#9Stick with a color palette of maybe 4 colors tops. It will help your room look more cohesive rather than small and cluttered, not good when it IS small.
#10Remember stores like IKEA may come off as a bit to conformist to some, but some ppl decide to screw with that. You can alter whatever furniture you find to suit your needs with some time and a little bit more money. Try http://ikeahacker.blogspot.com/