This may have been discussed before in the past, but it's been in recent news again.
A SITE Intelligence Group summary from back in 2011 said jihadist forums contained several discussions on ways to beat thermal cameras. Some of the strategies included wearing cold wet clothing, or getting on all fours so it was harder to distinguish a human from an animal, or making a suit of mud covered sacks to wear. The summary has since been taken down (or I'm just having a really hard time finding it online).
I don't think any of these would really be great at "beating" thermal cameras, but I'm wondering if they would be effective enough give someone difficulty making out an image. I talked to a couple law enforcement guys with experience with FLIR and got mixed answers. One said maybe. One said absolutely not.