I'm not specifically using a system, just laying all favourites in fields of 10 or more.
I listen to the track odds come over the net radio just before the off, and lay the favourite if I can get less than the bookies on track price.

Last night there were strange movements indeed. Oncourse a horse was 11/2 Favourite and I got $3.40 on Betfair. That horse went into $2.80 then blew out to $5.50!

So after cursing I stood to payout too much, I actually got a much better price.