I'm not going to lie to you -- food shots are hard, and they generally involve a lot of lights (not power, quantity of sources). And one way or another, this is going to involve alcohol abuse*. Forget flash altogether unless you've got a friend who can lend you a set of studio strobes with modelling lights. You can use lamps (if you have a work light with a parabolic reflector or two, all the better) -- either incandescent or a good fluorescent will do. You can use waxed paper as a pretty decent diffuser to make the light softer (watch it if you're using incandescents).
You're going to have to spend some time looking at the shot as you set it up, adding reflectors and gobos (basically, bits of card to block light, usually matte black to minimize reflections). Most of this can be bits of card, perhaps with a bit of aluminium foil (if you have a ready supply of the embossed foil from the inside of a cigarette packet, that's a great, even reflector surface). Don't get too fussy making things that don't show in the image, but do worry about anything that will be reflected in the glass.
You'll want to work with just the glass first to get the right highlight shapes. Then for the painful part -- you'll need Guinness in the glass while you try to get the nectar to look like something other than a dark, dense gravy with mashed potatoes floating on it. That may mean throwing some light into the liquid from behind -- a small torch with a teeny, tiny "striplight" (a rectangular softbox) that can be hidden completely behind the glass might be appropriate. Take your time and play with the light -- you're not going to shoot the first pint since all the foam streams and the gentle collapse of the head only look right for a few precious seconds, and this is going to take many minutes. Have a less-than-perfect pint as a reward for all your effort, and make sure that if there are any little folk around, they've been adequately frightened of the consequences of touching anything.
Take a break -- you will have been hunched over and unaware of how stiff you've become. Meticulously clean the glass -- like someone's critical surgery depends on it. Make sure everything is still set up properly. Now for the perfect pour, put the glass exactly where it was during the set-up, and take a few shots (preferably bracketed -- mains power fluctuations can do strange things). This should take only a brief few seconds this go-around. And your reward awaits you again.
*It's up to you whether than abuse consists of sacrificing perfectly good stout or getting yourself into a condition that isn't conducive to photography because you don't want to see it die an unnecessary death.
