Leave eggs in her nest . Natures way is for a hen to lay a clutch of eggs and then go broody .
The other thing I have heard is have a bunch of chicks in the area, the sound is suppose to bring on the mommy hormones.
Also I think fake eggs will help too. They become broody by the chest feathers being stimulated by eggs under them... Or bring them here, last year I had no broodies until July. This year everyone wants to go broody.