What CDR said of course.

I try to get my speed up and then sprint. Get really aero deep in the drops and accelerate smoothly, seated. Be your own kilo man. Don't sprint, just try to get it up to 30, spinning fast. If you're in any kind of shape, you should be able to hold this for a minute or two but don't just sit there at top speed, either. You'll want to be in the big ring. When you've hit your top speed, shift up to your 11 or 12 or whatever you find you can manage, get your butt out of the saddle, back still flat, and hit it for 10 seconds. Do ~6 of these, separated by 5 minutes of easy pedaling. The whole acceleration + sprint should take less than a minute.

I also do this another way: shift into your big ring and a smallish cog in back and slow to maybe 10 mph. Then come up and with your back flat, just accelerate as hard as you can until you explode or spin out the gear. If you spin it out, try a bigger gear next time. Again, ~6 with 5 minutes between.

Another useful thing is hill sprints: find a hill you can climb all out in about 45 seconds, not too steep. Come into the hill at about 20 in your big ring, hands in the drops, shift up a bunch in back, come up and attack. Try to find a gear that you can hold all the way up the hill. Again, ~6 repeats, 5 minutes apart. I'm much faster if I pull up like crazy.