OK, this is advice from one whos STILL trying, but I've made the mistakes, so at least i can steer you clear of many of them.

First 18% is probably a bit rich, nearer 15%

Now that can be reduced to about 106-110%, simply by always betting bets tote/SP, or if youre backing shorties most of the time bet at top fluc.

Ok so now youve improved your figure by about 10% just by betting at the right place.

Next step (so they tell me is to back at Betfair)which apparently means that the book is reduced to about 105%.

Ok, so now apply various filters that reduce the number of bets BUT increases the POT.

At the moment I enjoy around 4% POT but that is betting for the place.

OK so WHAT filters?

I'll start with my 3 favourites and see if anyone can add:

At least rated 97 (unitab) (this cuts out a lot of donkeys)
METRO run last start
No bet 3kg claimers/amatuers/ladies/jumps/2yo/races for Fillies and/or Mares , only, and any horse more than 7 yo and/or carrying more than 60kg's.

C'mon you lot add to that!