You'd be amazed at the difference that clip ins would make. It's not just the fact that your foot is connected to the pedal, but also the rigidity of the shoe. You could probably pick up a pair (pedals and shoes) on KSL or at Play It Again if you're persistent enough. Obviously, if you're wanting to buy used then finding the right sized shoes might be the biggest challenge. Be aware that there are several different cleat systems, but you can screw any type of cleat into the bottom of any type of shoe.
Buying cleats and shoes new would obviously be nice, but I'm guessing if you went that route they could cost you as much or more than you paid for your bike.
Joe is being awfully humble regarding his experience with cycling.
Joe - i have clip-ins I've never worn because I bought them before my bike (only have a mountain bike thus far). What bike(s) might you rec?