Leave the last seedheads on - they provide some protection for the winter. Don't cut them back at all, leave them alone until spring, when you can cut them back quite hard to new strong shoots.

thanks,can you split the plant up

I've answered yopur question on the other question you've asked.

If you cut off about a third of flowering shoots which have stopped flowering most penstemmon will flower until at least mid November. In a mild winter I've had them flower in December. After that follow Sprit's advicedow

Take cuttings if you want new plants. Non-flowering shoots root easily in water! Later, when the new rootballs are large enough, pot them up individually.

Whilst I freely admit to being a bit of a muppet when compared to Anchorman and Spritz, I have had considerable success with "staking down" fresh young growth. If the soil is compacted it's a good idea to give it a light hoe first. These offsets can then be divided April/May time.
If it's P.digitalis (see the link below), then it's easy to pull the new basal growth off and just pot up.

http://www.growsonyou.com/garden/show_garden_item/19598-penstemon-huskers-red

Ooooooo Thanks Spritz and Meanie for 2 new ways to propagate Penstemons. I will try them out.

Dorjac - this was asked twice. If you look at the other question, Anchorman will give you a third new way!

Iv'e counted 4 ways Meanie!! Potted cutting, cutting in water, layering, and earthing up......this is why GoY is so useful, with all these experienced gardeners, professionals and Open Gardeners being so helpful and as to share their experience.