I started an online betting account around 5 years ago with £20. I vowed never to top it up with more money and therefore stop once the money had gone, so I didn't end up losing lots of money on it.
5 years later, I've kept to my word and I now have around £70 on there. Hardly an earth-shattering profit but it's been a nice little bit of fun when I'm watching the football (most of my bets are in-play whilst watching matches).
Overall it's extremely hard to consistently beat the bookies as the odds are stacked so much in their favour. But I've found that if you take the bets you think are really good, rather than just taking a bet for the sake of it then you can edge ahead slightly.