Playing for fun is far better. Whenever I have tried to be calculative, I have lost. But recently I have just had a look over the teams and the matches and bet instinctively, and have incidentally started turning over profits, and have stayed away from the dicey matches. And as of today, I am in the plus, albeit small, but still.
As others have said, singles and at most doubles is the best way. Long shots bets hardly ever come off.
I remember how much stress I had during some games on which I had $100 or $150 on - even though it's not really a huge amount of money and I'll make it back at work in a matter of a few hours so losing it doesn't mean that much it still put plenty of stress on me. Nowadays I have $20 or $30 on a game it's not much of a stress, and I probably have a better hit rate since I changed my tactics! Almost got a 14/1 bet on Netherlands yesterday too only to lose because the amount of cards didn't click.
This World Cup is too good to be spoiled by bold $150 losing bets that but a damper on a game.