I simply take advantage of the aggressive marketing strategy used by many bookmakers and betting exchanges called FREE BETS.
I'll describe it using Bet365 as an example:
Bet365 has regular special promotions that double the deposited amount for sports fans: "100% bonus for you first deposit - up to £100" or "100% bonus - up to £50".
Bet365 in particular also offers such promotions to its existing customers on a regular basis!
(Almost all bookmakers frequently offer such promotions - check the bonus offers page for current information).
A hobby bettor who knows sports should take full advantage of this bonus - in the first case by depositing EXACTLY £100. Still using Bet365 as an example, their regulations specify that the bonus has to be turned over at least 6x before the resulting winnings can be paid out.
Therefore the trick is to simply achieve this minimum turnover through 'relatively safe' bets with low risk and in small amounts (not over £5); the only goal is to maintain the total capital as far as possible. Once you earn the right to a payout, you will have made a respectable profit of £100.
As I mentioned before, such promotions, betting bonuses, free bets are offered by nearly all bookmakers - some even repeatedly - so that a "full-coverage" network of betting accounts can add up to a tidy sum.
While it may be true that this is not as lucrative as, say, turning £10 into a full £1000 within a month by employing a sophisticated strategy, it is much safer - and also entirely legal.
Just consider it payback for the annoying continuous marketing strategy used by the bookmakers (and to some extent for the sometimes low odds).
By the way: In cases where a bookmaker only grants a small bonus - say £5 - it's not unreasonable to take a risk for a change. In such cases, I like to use one of the other betting strategies posted here!
Have fun, and fight the establishment!