I've been a Photoshop user for about eight or nine years now and I can trace it all back to two things, the gorgeous and extremely talented young woman who first showed me the ropes, and a Macworld book. Since the first one isn't an option for anyone these days (she's married now and keeping a low profile) I can only recommend very strongly The Photoshop Bible by Deke McClelland. It is the single greatest influence on how I figured out what photoshop can do and how I can make it do what I want. Next, check out Adobe's web site for tips and tricks. Then, remember to have fun.