To ensure you make a profit no matter what the result you need to ensure you have spread any profits made across all selections, this is done by 'Greening Up'
If you just 'Close' a trade you will only make a profit if the selection you have traded wins.
Its easy to check if you greened up or just closed a trade just by looking at your matched bets or log within Bet Angel or by checking you account on Betfair.
If you have greened up your back and lay bets will total a different amount.
If you just closed a trade then your back and lay bets will add up to the same amount.