Beer with good condition should not be tampered with. However, sometimes the beer contains enough carbon dioxide but fails to deliver a good head or any head at all. I'm not an advocate of a big creamy head, but I like to see some activity at the surface of the beer. Washing thoroughly to remove any traces of fat, oil or detergent is essential of course. Also scratching the bottom of all your beer glasses with (eg) a tile scribe is a clever way to help seed fine bubbles from the bottom of the glass. I've been through all my glass stock to scratch the bottoms. Also I have found that adding a very small amount of finely ground mixed spice powder to the glass (on the tip of a teaspoon) enhances the head and head retention of flat homebrew beer. Add a lot less than you think you might need. You can add it after pouring the beer if the head is disappointing. Gently stir it into the beer. It evidently seeds the little bubbles of CO2, provided that there is already CO2 in the beer of course. The flavour of the beer is enhanced with the hint of spice. Most of the powder does rise to the surface and make the glass and head look a bit messy, so add only a tiny amount at first. I've put the spice powder to the test...very small fine bubbles continue to dance around the top of the beer when the glass is gently shaken. When I used heading liquid the head did not seem natural...it floated away as you tipped the glass to drink. Better of course if you can get a natural head without recourse to spice powder and the like. I've heard that including flaked barley in the mash aids head retention, say 500g of it. I used to add this to my stout and the head was better than for bitter. Next brew I'll include some flaked barley for sure. I've heard that a little wheat malt is also good for head retention.
