I'm due for my 4th DVLA medical in 7 years for the same reason as I can only get my licence back at intervals as my results are inconclusive of misuse, maybe because of the following so be aware!
Anyway, I usually for 2 weeks beforehand don't drink at all or take any drugs, including paracetemol as these make the liver work harder but are not known to be tested for anyway. I have found that Milk Thistle available from health food shops taken 3 times a day lowers one of the enzymes (AST) the blood test looks for
but cannot find any method to lower the other two as these (GGT and MCV) are only high due to long term misuse and only 6-12 months alcohol free will reduce these.
The acceptable levels are quite high that the DVLA look for and as a heavy drinker I have only ever failed once due to the AST level being 2 points over their level and was well within the others. Also cut down on cigarettes and fatty food for the two weeks prior and drink plenty of water as the Milk Thistle needs this to clear the liver. I recommend 2 litres a day. Take multivitamins and eat fruit and try and get some exercise too. You'll be the healthiest person on the planet for a fortnight and you should pass the blood test, after you've parted with the £96 (as at 14/9/09) medical fee.
BTW - This pointer is for those who have made a mistake in their driving career like I have and 10 years on still being reminded of it, so please no nasty comments about drink drivers on this thread. Use another thread for that.