Unfortunately I live in a big mean city and like to keep my light in my bag when my bike is locked up. A problem I've frequently encountered with my light is that it will randomly turn on in my backpack far too easily which drains the battery. Very frustrating.
I was able to solve this problem quite elegantly with Sugru.
1x - planet Bike rear light
1x - packet of Sugru
Step 2: Apply Sugru to Bike Light
Make sure there is empty space between your sugru dot and the bike light switch! This is what lets a finger push to turn on the light and not books in your bag.
Step 3: Let Sugru Set
Leave to set for 24 hours.