Google scholar BETA is an excellent source IMO (at least in my case).
It actually finds much more citations than what I could find from
other sources. Only problem is that it sometimes does not point to
official prints of papers that cited your paper.
You'll need to submit offical prints or copies of research papers
that cited your work. Google scholar instantly trippled the # of
citations I got from other sources initially but the entries had some
papers that are not truely internationally circulated or their
official copies were not easily obtainable...
(But makes you feel not bad knowing your work is being cited
by stangers in very obscure places in the world...

)
If you're in CS or EE related area, you probably know you have
ACM portals and IEEE Xplorer which has all the official copies electronically...
Good luck,
fdfd