For a cyclist a large aerobic engine is essential. When you go for a ride you stimulate your aerobic system, but how much stimulation depends on the intensity you ride with. Trained cyclists need a greater absolute and relative workload to keep improving the VO2 max.
Thus, if you are an experienced rider, it is necessary to do hard intervals or races to make further progress. Elite athletes will sometimes reach a plateau where there will no longer be significant increments. At this level, the riders train to work at VO2 max for a longer time.
Maximum power test
As I have described previously in How to train with a power meter for VO2 max, it is a very good idea to do a 5minutes all-out test to find your average Watt at maximum oxygen consumption. There are other tests, but I think that this one is easy to use and still very reliable. You can do the test on a stationary bike or the road if you are the lucky owner of a power meter.
Killer intervals for VO2 max
All intervals should be done with an intensity found in the maximum power test.
3 x (3 + 3min)
This one is a winner. Done correctly you got 9 minutes of effective training at your VO2 max. You can make a little twitch to this one by making the recovery period shorter. That will give you more time with maximum oxygen consumption.
5 x (40 + 20sec)
I like this one very much even though it is not as effective for your VO2 max as interval program no. 1. It is though very good for race preparation and is very often used in the personal training programs I make for my riders. Normally I recommend 3 to 4 sets.
1 x 5min
This is the hard way. I do not recommend it, but it works.
12-20 x (30+30sec)
This is my favorite workout if you want to boost your VO2 max quickly. Based on scientific studies these intervals are some of the best in theory. But like in all other training situations it is important to be motivated for the thing you decide to do.