Those of you with those exercise balls out there...

...anyone got any guidance on a proper way of doing stomach crunches on them?

I've just done 200 and fear that my head is going to split in two with a headache...

I was exhaling on the downstroke (as it were) to minimise it, but it has just wrecked my head, even though I know it seems to (at least feel) like its doing great things for my stomach...

Madness...