The best, fool-proof way to add flowers to a room without looking over-the-top is to stick with one type of flower in one color. This gives a more modern feel. Lose the greenery; it can make the arrangement stuffy if not done exactly right. Select vases that suit the flower.
Here are some images of my favorite flowers to inspire you:
This tulip arrangement is gorgeous. Since tulips stems bend over time, I love how this keeps all the blooms right together for a color burst.
Roses have a strong fragrant- too much for my liking. This vase holds five blooms which is just enough to scent a room. This would look great centered length-wise on a coffee table or in front of that horrible television set we always try to hide.
http://www.countryliving.com/ This arrangement of dahlia's is gorgeous. It is feminine, southern, and sweet; everything I love. I recently copied this arrangement for a girlfriend brunch centerpiece. 
This is a pretty horrible image (but the best I could get in my nearly-no-light bathroom) of a gorgeous combination of flowers that I have in my bathroom right now. (Also ignore the horrible faucet. Since I live in an apartment I am stuck with it.) The great thing about this combination is that the lily buds provided nice greenery and by the time they finally started to bloom (about a week after I bought them) the daisy's had started to wilt. I threw them out and had a nice, white arrangement. And I got that fabulous vase at Wal-Mart. (Yes, Wal-Mart!)
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