White noise machine. Works great. I can still hear some floor creaks when my upstairs neighbor is walking around, but not nearly as much as I used to. My bedroom window overlooks the parking lot for our condo courtyard but I don't hear any car noises. I am still able to hear my alarm clock and also the cell phone on my nightstand if it rings.

The one I have is a small clock radio/cd player that also makes a few different nature sounds. The babbling brook and ocean are nice, but the volume is not consistent so they don't work to mask neighbor noises. But there is a rain option that is very much like a constant static noise.

I know some people just use a fan, but the white noise machine is louder and therefore works better for me.

I also have insulated drapes in my bedroom that block almost all light from coming in, if that is an issue. I got them at Target -- I think the brand was Eclipse.