Are they in large containers or in the soil at the moment? Then wait until autumn, pot them in ordinary multi-purpose compost, and nip out all the remaining flower buds. Then keep them on the dry side in a frost-free place until the spring - watch out for any signs of botrytis (grey mould) and remove the affected leaves immediately. Some people cut them back, but I've found that they survive better without doing that. Never water from above - the splashes of water on the leaves will encourage mould. Don't crowd them together, either - that can also cause problems - they need some air circulation, and don't feed them.

In the spring, repot and start them off again.

Interesting that you recommend not cutting them back, Spritzhenry. I know this last winter was exceptionally cold but I cut all mine back ...for the first time ever...and they all died!

Luck of the draw sometimes, I reckon - I cut all six of mine right back to about 2 inches in early November, stuck them all in one pot and shoved them under a seat on the balcony, up against the wall - all came through and are now flowering merrily away, bar one.

I kept mine in flower all the way through this winter as an experiment! Amazingly, it worked this time. Normally, I'd say take off the buds to allow the plants to rest and conserve their energy. I have also cut them back, as Bamboo did, and lost some each time.

I kept perlargoniums in a small plastic shelter meant for a grow bag... they were just shortened slightly and kept in flower. I had some in very large pots under a double fleece tented arrangement...closed with pegs at the top, so easy to inspect at intervals.....they survived and one pot of three has been has been truly spectacular in this hot weather. The pots were in a close huddle to try and keep frost off roots and save space on the patio under a verandah. Two large fuschias survived in same way, one was lost. The tent was done only as the cold weather started in December. You may need to use a Provado drench, if you do this, to kill vine weevil grubs in larger pots. Smaller ones can be emptied and picked over.