If you think your room is too plain or even boring, it's easy to liven it with just a few easy steps.
It's all about the little things.
Of course you can go ahead and paint your walls some non-standard color and that certainly can change the mood of the room that the walls will project, but you don't have to do that, just add a few little details that will have it shine in a new light.
Personally -- and of course it's very subjective, pick what best suits you and your habits -- I think accents on walls and plants make the room lively and give it that inhabited look.
It's really up to you what to put on the walls, it doesn't really matter, although I, myself, prefer functional things.
It can be a poster, a reproduction of your favorite artist (or an original!) or a photograph, it can be a cool clock.
The only thing to keep in mind is that whatever you put doesn't need to be expensive, but it can't be tacky.
Your room tells a lot about you, so it's best to keep things simple and tasteful.
Plants can really add life to any location and they aren't that hard to take care of.
All it takes is just one or two and you won't believe how much a ficus can change the whole atmosphere.
Besides, they are living things, too, so not only will they make the room cozy and inviting, they will also announce that you are caring enough and can take care of living things.
To give you a couple of starting pointers, I would go to IKEA or Home Depot, for plants -- they have a decent variety at very affordable prices; for prints, I would go to a site like Photo.
net.