I use this system on Betfair, its blindingly obvious but took me a year to twig.
You need a minimum bank of a thousand times your laying off fee i.e lay off for £2 down you need a bank of £2000. Just to cover yourself.

Lay off the the lowest priced horse in the race for £2.
It wins you lose £6, add £2 for the profit you should have made, equals £8. Divide this by two = £4.
Layoff the next horse for £6 (£4 loss plus £2 profit)

Now if the horse wins add up the total amount you've lost divide dont forget to include your £2 per race, and divide by two for the next race.

When you come across a streak where two Favs lose you then have your profit and stake money back.

Bewarned !!! Quite often a streak of 6 Favs will win, you need plenty of money to carry this through but believe me it does work.