There's a whole shitload of folklore out there about flushing. First of all, you DO NOT flush plants growing in the ground. You DO flush plants either in pots or grown hydroponically. Its very simple. Use plain ordinary water about 10 days or so before harvest. But if you want to spend money, then be my guest. You can find a myriad of products for good bucks on the web. Using H2o will give you a smooth smoke afterwards, providing of course that you dry and cure properly.
There is a molasses product around which claims to 'sweeten' the buds. I don't believe it.