mumslife, that is so like my ds1, especially the not seeing why he needs to get washed and dressed at the weekend and setting the time he will do something and it not being able to be a second before, but anytime after said time is fine by him. 
I think switching the rewards every now and again is a good idea to keep them motivated. As I said, ds initially did it to 'earn' extra minutes on his DS, but now he's keen as mustard to get ready in the mornings because it means he gets to use dh's laptop to go on Mathletics.
The ace in my pack is his desperate desire to play games on the website, Nitrome - but I keep that strictly for times when maximum motivation is required.
It had a dual purpose, in that because we only use it rarely, not only does it send a huge message of appreciation/affirmation for whatever behaviour he's done, the games themselves are also a massive reward for him.
The motivation thing is all basic behavioural theory really. I remember from when I was studying dog behaviour that all rewards become jaded after a while and therefore become less motivating. Eg if every time you pulled the lever on a one-arm bandit you won the same amount of coins, eventually you wouldn't bother anymore, whereas if you don't know when or how much you might win and sometimes you get nothing at all, the drive to pull the lever (ie the motivation to try harder) is stronger. You need to change the rewards about, up the ante sometimes and ultimately only reward the best behaviours in order for it to continue working.
With dogs you have several levels of reward, with A grade being the thing they desire most in the whole world (for my dog that is her squeaky ball which she is rarely allowed to play with) and that's the one you use least often, but make sure you throw in occasionally to keep their interest up - in my ds1's case, his A grade reward is going on Nitrome.
Sorry, I'm being boring now. I'll stop. 