Wait till the frost blackens the foliage ,Mouldy,and then lift them..Get as much soil off as you can,trim the stems back to a couple of inches..and dry them off till all the moisture has gone..I used to stand mine upside down,on the stems,so try to cut them to equal lengths..some may do differently of course,but it worked for me..
When they are perfectly dry,store in paper bags,somewhere frost free..I put the bags in a container..
but keep checking for mould,Mouldy ! Lol..sorry :o}
the paper bags may feel a bit damp,so replace them as and when..hope this helps,and I'm sure you will get more advice ,on other methods..good luck :o)

I've stored them in dry compost - old stuff will do. Just keep off the damp as Bloomer says, and also put them where mice can't get at them.

Mine were kept in the top of a Wardrobe :o)

When the frost blackens them, do not lift straight away.
Cut the stem to 2 " above soil level and leave for at least 2 weeks. This encourages next years eyes to form.
When I dig them up, I always wash with a jet spray from the hosepipe to get rid of all soil and trim off the fine roots.
Then leave to dry in the greenhouse for a week or so. Store them when dry in dry compost as above, in boxes, buried up to crown level. Store them anywhere frost free that has good air circulation, but doesn't go above 50'f.