I limit myself to £5 a month on SkyBet (always guarenteed to get "5BONUS" too making it a tenner).
If I have £10+ after a few bets I bank it and wait another month.
From 01/01/2013 to 31/12/2013 I deposited £75 (introduced limit into the year) and withdrew £71.25 so not bad spending £3.75 on a bit of entertainment.
So far this year it is deposited £10 (plus 10 from bonuses) withdrew £24 so £14 up.
Overall a profit of £10.25 in 13 months, check me out.
Like others have said my downfall is getting bored and betting on stuff I know nothing about (e.g. X is beating Y in tennis, put money on X even though he may be ranked 100 places lower and just started well). That and the slot machines.
I had a big win on online slots recently and closed my account after that so I didn't go back. Most of that went on sports, went up and down for ages, lost patience and went for broke.
They do say gambling should be about entertainment and not a way to make money. You've done it exactly the right way.
Since 18/07/2011 (when I created the account) I have:
deposited £238.50
withdrawn £255.95
So a whopping £17.45 profit after 2 and a half years.