Don't be afraid to get down low to the ground. I think bikes look a lot better when shot from a low angle. Anytime I've ever shot motorcycles from above it always turns out crappy. Maybe it's just me.

Next, find the right location. Don't go to where the bikes are if you can avoid it. Bring the bikes to you. Put them near a beach, in front of a waterfall, or maybe in front of an office building. The key is to make the background pleasing and then put the bike in it.

Last, go on Flickr, search for motorcycle and get some ideas from others.