Have at least a treatment, better yet a script, and register with the WGA (www.wga.org). Keep a log of the people you talk to and show your work to, and start networking. Check out Jerrol LeBaron @ Writers Script Network...
Sometimes the only useful thing an agent does is document your meeting with who ever you pitch it too...who you met with, time, what you talked about..etc, so for that reason, you might consider it, except they get a good chunk. Just to keep this on topic, how about, use vegas video to document your pitch, video tape yourself going into the meeting with shots of your watch, that days newspaper, and your certified treatment...seems a bit paranoid. You may recall that Mark Dunne (ella minno pee and welcome to higby) was awarded undisclosed $$ for writing the Truman Show, which was blatently ripped off right after he pitched it to the same producers who made the movie...or something like that.