Gadgets: Buy a u-lock (harder to cut) rather than a chain, wire or some other configuration. Buy a cycle computer (shows you your speed, distance for your ride, etc) - the feedback is great. You'll know when you're pedaling slower than usual and also can keep track of how far you've been riding. Last of all, buy a pump for the tires. Keeping your tires properly inflated helps a lot.

Contrary to what has been suggested before, I would buy the lightest bike you can in your budget. A lighter bike will reduce the agony when you don't want to push yourself; and when you do want to push yourself, just go faster.