I agree with the posts above.
No need to do anything over the top. My diet / exercise regime goes up and down all the time. I've had weeks where i'm down the gym every night and eating meat, fish and veg (all the good stuff) then weeks where I'm down the gym for 30 mins a week and eating a pizza and whole tub of Ben & Jerrys for afters! I was fine on my medical. One piece of advice is to drink plenty of water 24 hours before the day. I drank hardly nothing and downed a litre of Pinapple juice half an hour before I went in. Needless to say my first Urine sample wasn't up to standards and 2 hours and 32 and half cups of water later I was fine!
I agree with the alcohol comment. On your Medical form it'll ask you for weekly alcohol consumption in units. I believe the CAA pass bracket is somewhere within the 15-24 units a week.... Anything less and you need to get your ass down the pub more!!!
Anything more and you'll half way there to the left hand seat.
Just kidding!