Still can't get the tension on the drive belt right,anyone help ?

I've followed all the instructions,the belt is on - but after hours of fiddling i still can't get the tension right - i've even changed the spring on the motor from the left to the right and back again - the belt slips against the drum when turned on so i know it's just the tension,can't help feeling there's just one simple trick to this - can anyone suggest anything ???