Does your leg go to sleep and tingle, or does it physically twitch or move involuntarily?
If it is a mere sensation, then I would suggest just trying to monitor it with mindfulness. But if it's a real movement that you can't control, you're probably putting pressure on a tendon and that's not good. I'd recommend using the Burmese position, as Ben said, or getting a stool.
Also experiment with moving yourself up or down. Sometimes a folded pillow under the cushion makes all the difference, as a higher seat allows for deeper hip rotation. This may solve your problem too.
Gain and loss, status and disgrace,
censure and praise, pleasure and pain:
these conditions among human beings are inconstant,
impermanent, subject to change.
Knowing this, the wise person, mindful,
ponders these changing conditions.
Desirable things don’t charm the mind,
undesirable ones bring no resistance.
His welcoming and rebelling are scattered,
gone to their end,
do not exist.
- Lokavipatti Sutta