Often its just trying to have something in the foreground as it is very easy to be too far away when you don't feel like you are far away from what you are putting in the foreground.
Watch your toes to make sure they aren't in the shot, you'll be amazed at what you can fit into an 8mm shot. Go out and take many, many, many photos. It's a fun lens for both city and landscape.
Get cloooooose-up! Read Ken Rockwell's article on how to use ultra-wide lenses. The differeces between three feet and one foot can be very monomental on lenses like the on you're buying. And keep in mind that 8.9.10.11.12.13.14.15--they seem close, but there's a LOT of difference between 8 and 10, and 12 and 15. Personally, I find the 8 to 10mm range looks very "affected" in many photos.