You can't stop any lavender from becoming woody and leggy over time, but you can delay it by clipping them over after flowering, removing about an inch of foliage (less if the plants are small) at the same time - expect to do this twice a year. Average time till they need replacing because they've got leggy and ugly is 5-10 years.

Any stem cut below the foliage isn't going to sprout out, though.