1) Ride, ride, ride. The more miles you put in, the more you will improve your speed and endurance.
2) Interval training. 10 minutes at a moderate pace, followed by a 10 minute sprint. Repeat. Do this for an hour or so every other day, 2-3 days per week. After a couple months, you'll definately notice a difference.
If you're not in pretty good shape, stick to #1 - intervals are tough, and probably not such a great idea if you have health problems (especially any sort of heart condition). As always, consult your doctor before beginning any rigorous exercise program, blah blah blah, etc.
'08 Bianchi San Jose
'06 Giant OCR C3
'04 Specialized Hardrock Comp