a few more tips to calm a wobbly bike

RELAX your upper body - relax your neck and shoulders, elbows slightly bent and used like a shock absorber - hands always around your handlebars, but no death grip. When you are stiff in the upper body some of the motion of your legs, instead of all going into propelling the bike forward, transfers up through your body and into your handlebars, causing your front wheel to wobble back and forth. So if you relax, not only will you not wobble so much, you'll go faster too. This is probably the biggest cause of a wobbly ride that I see in new riders. It will creep in on you too - so even if you start relaxed, take mental stock occasionally - bring your shoulders down, bend your elbows, loosen your grip.

Make sure you saddle is not up a little to far - if your hips are rocking back and forth over the saddle that can cause some wobble too.

And in the spirit of #1, make sure the bike is not too long - if you cannot reach the bars without having your arms straight out, elbows locked, your top tube is too long. If its only a wee bit too long you may be able to fix it with a shorter stem, but remember that a teeny tiny stem can affect handling too, so if you are finding that you need to go to a 5cm stem, its probably time to think about a smaller bike.