How do you dampen street noise if your bedroom window is above a busy street?

I find that you can't really dampen street noise, but you can more easily cover it or drown it out with white noise, like a fan.

If you own the place and are willing to invest some money, you can buy double-paned windows, which make a huge difference. If you're a renter, you could try convincing your landlord to invest and buy them for you, but that might be tricky. Other than new windows or a fan, earplugs might be your best bet.

It seems there are only a few incremental changes you can take with street noise.

First, I'd start from the outside. Is it possible to run some climbing vine our plant up the wall outside of your window so that it can become bushy or full enough to absorb some sound on the way in? Do you have any planters, boxes, fire escape outside of the window to start growing a barrier? Second, I'd hang heavy drapes, potentially with some discreet padding hidden behind them, as a second line of defense. Then, I'd try to cut the remaining noise with the fans the other white noise referenced above.

whoops...forgot to add...and...it may also be helpful for you to contact a really high-end hotel near you to see how they handle it. They may have very specific items they use (in terms of drapes and padding) and they can give you the name of their manufacturer...

I was thinking along the lines of a small fan or water fountain. But there is a cottage industry for such annoyances without have to go into some heavy remodeling.

Sound conditioners are fairly inexpensive and may offer a better masking of the sound than the formerly suggested products. Here is the manufacturer that popped up on a Google search; http://www.marpac.com/sound_cond.asp