VO2 max is basically genetic. training will only improve it by about 10%. If you've been training for a number of years already then VO2max is unlikely to improve much further.
another point is that VO2 max starts to decline after the age of about 18.
bad luck

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regular training stops the decline being quite so rapid though.
what is debatable though is how useful VO2max is as a fperformance predictor - most research suggests it's pretty hit and miss.
Performance is predominantly determined by how hard you can go for a prolonged length of time. this is level is known as the anaerobic threshold (AT) - go much above threshold and lactic acid will accumulate rapidly and you'll have to slow down.
whats nice to know is that training (especially specific interval training ;D) will improve your AT.
think about it like this.
2 riders, rider 1 has a VO2max of 80ml/kg/min and his AT is at 50% VO2max. that means he can ride comfortably for a long period of time using Oxygen at a rate of 40ml/kg/min
rider 2 has a VO2max of 60ml/kg/min and his AT is at 90% of VO2 max. He can therefore comfortably ride for a prolonged period of time using O2 at a rate of 54ml/kg/min.
who's going to post a better time in a 40km time trial?
yep, you guessed it, rider 2: despite his lower VO2 max, he can maintain a much higher pace than rider 1 due to his higher AT.
If you want to know all the physiology behind VO2max i could probably explain it to you but to be honest, it's pretty superfluous to what you really need to know - that VO2 max doesn't improve much with training, but AT will.