Don't bet the favorite... take two horses in each race that you think has a legitimate shot of coming in first or second (if the fave is one of them, skip the race) and wheel those horses up and down the card... for example, if you like the 1 and 2 and are giving you decent odds, wheel them up and down the card like this:
1/all, all/1
2/all, all/2
I have hit some very large exacta payouts doing this, all you need is your horse to come in with a big longshot and you'll have yourself a very nice payout coming your way.
EDIT: Just want to add, I made about $300 on Derby day doing this. It's the only way I play horses now.