Say today you join Sportingbet and deposit $30-, you will get a $100 free bet for joining.
Option 1
You can put it straight on a Line Bet say Adelaide -10.5 $100 (free bet) @ $1.91, if it wins you will get $91 profit.
Then go to another bookie or TAB and back St Kilda +10.5 for $50 @ $1.91.
Adelaide win you win $91 at Sportingbet and lose $50 at the other bookie = $41 profit
St Kilda win you win $45.50 at other bookie and lose nothing at Sportingbet (free bet remember?) = $45.50
cant lose

Option 2
You need Betfair for this one, but can make more from your free bets with this method
So you have your $100 free bet at Sportingbet, you now ned to find a close match between Sportingbet and Betfair.
Lets say Richmond tomorrow are paying $8.00 at Sportingbet and you can LAY them at Betfair for $8.20
$100 free bet on Richmond at Sportingbet @ $8.00 = $700 profit if wins
Now you LAY Richmond at Betfair for $8.20 for $86- (Liability of $619.20)
Sooo if Richmond win you win $700 at Sportingbet and lose $619.20 at Betfair = $80.80 profit
If Richmond lose you win $86 at Betfair less $4.30 commision = $81.70 profit
So as you can see with Betfair a free bet can be turned into about 80% of its value in cold hard cash
any questions feel free to ask....