Hi Raven,
Welcome to the world of lay betting.

What you will find , is no matter how good one is, at selecting the horses that should fall over, they will still get up 10-15% of the time.

Even 20/1 shots will get up 1 in 20 times.

I have found its best to try & target runners at $7.00 & less, that way if one gets hit 1 in 8 times , a profit is still to be had.

That's a success rate of 87.5% SR, its very difficult to get much better than this over say 100 bets.

Turn over is pretty much the name of the game because of the low odds one is working with. So the idea is to have heaps of bets if possible.
It is often suggested to bet 1-.50% of bank.

Try not to increase bets after a loss , but halve the bet if the race or prices looks risky.

Halve your normal bet on $11.00 shots
Quarter your bet on $20 shots.
Because when they bite , they bite hard & its a long grinde back up the ladder.
That's just the nature of the beast.

Cheers.

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Cheers.