Get down your chord progression, and then just tell a story. If you can record it, just loop it and scat, make up stuff as you go along that you thing sounds good. Have a pen and a pad handy or record your voice. Talk about how you were at a record store buying Parliment CD's and you saw a girl out of the window that looks like Anna Nicole from the early 90's. Talk about how your friend spilled beer on your dog and your cat was all pissed off cause it was in heat...
I don't know how everybody else feels, but I don't think lyrics really matter as long as it sounds good.