I invariably semi-sit on the back of a chair. It takes the weight off my legs while leaving the sola plexus unconstrained by a folded midriff. The first half pint is good for nerves too, but not necessarilly the next few. Knowing that the reason for shaky legs (and the larynx as that is a muscle too) is adrenaline also helps - you know what to aim for in reducing the effect. Alcohol lessons that. Telling a joke before you sing (short ones) can help get you and your audience started but chose jokes wisely, humour is all about context, if it falls flat it will unsettle you more! Chose chorus songs a lot - it helps when they are singing along to prepare for the next verse (another adrenaline moment).