I've done the same as tomtruck. If you can mount the PIR so that the distance can be adjusted the sensitivity can be "tuned" for your conditions. For example during the summer when the air temp is close to body temp, you want to improve the sensitivity. During the cold months you want to reduce the sensitivity. I mounted my PIR board on some long machine screws with multiple nuts.
I understand there is some material available from a company called FresnelTech that changes the IR transmission properties of the lens.