There is a mathematical way of reducing your probability of losing, though nothing can ever guarantee a win.
(Basic Rules)
1) If you are betting each way then never bet in a race with less than 8 horses. Races with 4 or less runners the horse has to win, 5, 6 or 7 horses, it has to come first or second, 8 and more it can come in 3rd and you will still get something back.
2) Only bet on Hurdles races...your horse is less likely to fall.
3) Assume a £1 each way bet....that costs you £2. If it wins then you make a profit.
If it comes second then you get a quarter of the odds...example the horse is 4 to 1, you get £1 x a quarter of 4 to 1 which is 1 to 1 or evens. That means that you would get your £2 back.
If it comes third then you get a fifth of the odds, so to get your money back it has to be at least 5 to 1.
So back a horse at 5 to 1 in a 8 horse hurdle race......and statistically, the longer distance races are won by the most favoured horses, e.g. favourites or second favourites, though don't discount that old cart horse that was trained by Chubby Brown jumping over piles of rubbish in the local tip....
Good luck!!!