One of the most effective workouts I do on a regular basis is the Coggan Power Interval. This has proven to drastically improve my power output, more so than any other single workout I have ever done. I did a LOT of searching on the internet before I found out what seems to be the preferred method for this effort.
If you don’t know your Power Threshold, then you need to determine that first and foremost.
There are a couple of methods, but what I used was a 20 minute ride at maximum sustained power output. Ride 20 minutes as hard as you can maintain, and then take your average power output from that effort. This should be your Power Threshold. The Coggan’s Intervals are very much like this test, which is why they are so painful.
You basically do 2 twenty minute workouts at between 90% and 104% of your power threshold with a five minute recovery in between. This allows you to maximize your power output during your workout without too much lactic acid and therefore allows your muscles to get their maximum workout.