Simple answer -- I don't. I've been working out regularly going on six years, now, and I still can't balance during lunges. I can't even balance during unweighted lunges, so I don't think it's a lack of strength, really. I've always had ear/balance problems, and I think that plays into it.
I either hold onto a cut-off broom handle with one hand and a dumbbell in the other, or I put a weight in one hand and hold onto a workbench with the other. I've tried and tried and tried it without a balance support, but I gave up in frustration. In order to get a good leg workout, rather than constantly falling over and trying to catch up, I just hold on to something and concentrate on using the support only when absolutely necessary.
Shari