As soon as they have flowered cut off the blooms and new side shoots will start and again bloom. Keep doing this. Snaps don't like hot weather so may go through a period where they will just sit there but when it cools off they should start blooming again.Good luck. They are wonderful flowers.I have some of the rocket whites in my white garden and they are very slow to flower this year and I'm sure it's because of this awful heat.

and remember to just shake out the seed pods into the bed so they reseed. don't cover them as they need light to germinate

Addendum to witsend22's comment: Don't leave more than one stalk here and there to go to seed on non-hybrids. Plants maturing seed are more inclined to shut down and not produce new flowering spikes.

Another year, plan to get the snaps in as early as you can to take advantage of the cooler days. If they get strong growth started, they're more like to withstand the hot-spell-slowdown mentioned.

addendum to addendum (lol) I try to cut back all flowering stems throughout the blooming period leaving only a few near the end of the season to go to seed. I also like to mulch mature plants in the late fall early winter and then in spring I trim off the dead limbs and cut the plant back to about six inches.

deadheading the spent blooms also helps to keep the plants more compact.