Ahh, I've been doing the same kinds of experiments myself lately. To go backwards as well as forwards, you need to build an "H-Bridge". I've built 3 or 4 different designs of H-bridges in the past week. I recommend this one:
http://www.bobblick.com/techref/projects/hbridge/hbridge.html
I've got a couple of those built now. I'd recommend building it on a breadboard first, because I know I sure had a lot of trouble keeping my Bases, Collectors, and Emitters straight on perfboard until I had the circuit built on a breadboard and could just clone it onto perfboard. That H-Bridge uses maybe $5 worth of parts, and is good to something like 8amps. Point the backs of the darlington's outward, so you can put heatsinks on them if they start to produce heat, that will allow you to get a couple more amps out of them. 
Good luck.