My friend used to just hang them in her garage in bunches, upside down and it worked a treet, she then used to provide our church bazaar with the most beautiful dried flower baskets.

This site explains a couple of ways to dry hydrangeas, just copy and paste.

http://www.hydrangeashydrangeas.com/dryingnat.html

Have you ever considered buying the Ezidri food dehydrator ? it will dry all your favourite fruits and veg,saving deep freeze space,and with the extra trays added will also dry flowers.Look up www.Ezidri.uk .com and you can see a ten minute video which shows all that it can do. It is also on eBay,but if you get tempted,contact me personally.( to save money )
It could also be worth experimenting with a microwave I think,although I have not tried it myself.

My friend thinks the microwave drys them too quickly, they need, she says, to be done fairley slowly in a nice dry space, like the garage !