Take it from someone who works in a bookies: gambling is a mug's game.
If you've got to do it at all, bet small and aim big. A 9/2 for £35 or £150 on 3/1 Fav makes no sense. £1 on a 55/1 scorecast however, does. The difference being that you'll never miss a £1!
Don't chase your losses, and for crying out loud: stay off those damn machines! Despite what you think or hear from that idiot down the pub, you will only ever lose if you play them. I see people lose more than I make in a month on a single spin on an almost daily basis.
If by some chance you put in a £1 for fun with the intention of walking out anyway and win £100+ then yes, you're a winner but this is by far the exception to the rule. 95% will win on those machines and walk straight back over to them and put all their winnings straight back in.
I never bet before working for a bookmakers, but I don't mind the odd football accumulator these days as I know what I'm doing. £1 for a potention £3k+ return is fine by me, especially if I've got to sit and watch SSN on a Saturday anyway; it's a little bit of harmless fun. If I were to throw £40 onto a treble, it's not really harmless anymore (although it's all relative).
I don't want to sound like I'm mothering people, but I deal with the gambler's mentality every day; they don't want to listen so that's why I'm repeating myself.
Be sensible.