The "set" tasks that we have stickers for, for each child, are:
-Making your bed in the morning (usually DS's sister helps him out here)
- Sleeping through the night and staying in your bed (DS is a HORRENDOUS sleeper and DD has started to copy him for a bit of attention methinks)
- Cleaning up the the toys in the playroom/bedroom

Other stickers are given for different things depending on the day and the behaviour.

Eg lately DS has been kicking up a real fuss about having a pooey nappy changed so if he does it immediately without screaming the house down he gets a sticker. Or the other day DS spilt his milk on the couch and without prompting went and got a cloth and attempted to clean it up - was very pleased with that independent thinking so he got a sticker. Both children just got a sticker an hour ago because we returned from my SIL's with minimal fuss after having a lovely playdate - there were a few children there all of whom were horrors (well behaviour wise anyway!) but my two were fantastic and didnt get involved in any of the affrays and left without fuss when I told them it was time to go. So I chose to reward their behaviour.

If either of them constructively help me with a task that I request of them that is outside of their normal expected behaviours then I will give them a sticker too - this may be something like helping me fold the washing and put it away (DS trys to help with this so I do encourage it), helping to water the garden etc.

Hope that helps a bit.