alex cherney put me onto the new packs first, after previously not having much luck with the gel type warmers which didn't last long at all.
the new ones are a dry powder, quite light weight with a claimed heat time of 8-12 hours. they only cost a few dollars.. a lot of chemists have them now and i've also seen them in aussie disposals. they're not reusable though.
i find they take a very long time to activate.. about half an hour and they need a lot of shaking and flexing to work air through the mixture. sometimes i also think that when they're left on the lens they don't work as well as their intended use next to the body where they keep getting moved around which helps keep air moving through the mixture. alex seems to have had reasonable success with them though.
eg.
http://www.hotteeze.com.au/products/hotteeze-regular-1p
pack of ten for $22.95 or check out your chemist etc. i think the exact same product gets marketed/packaged under many different brand names now.
cheers
Phil