H.W.
I am not a big believer in long-term change with rewards (and there's a lot of evidence to back this up), so I use them with my son VERY sparingly. What I have found which works for me is sporadic, spontaneous rewards. Usually these are 'after the fact' things: example, I send Kiddo to his room to pick up and he does such a quick job and doesn't gripe, I might just say "Wow! You really got it done quickly and all by yourself. This looks great! Now we have a few extra minutes-- want to walk to the cafe and get a little treat/ we have time for a game..." Whatever works for us in the moment.
When we have a specific behavior that is driving me batty, I might do something different by offering a 'challenge'. A few weeks ago my son was getting into a habit of arguing with me and I had a tough day coming up, caring for a friend's infant. On that day, I drew ten circles on a piece of paper. Then I sat down with Kiddo and had him notice the circles and count them. "Each circle means a penny. Every time you argue with me today, I'm going to cross a circle off. However many there are left at the end of the day will show me how many pennies you get." Out of ten, we kept five-- but the great thing was that by the late morning, all I had to do was ask him "oh, are you arguing with me?" and he'd change the subject right away. After seeing five of them crossed off, he knew I meant business!
I'd also say keep it simple and keep the pay-off small. Lastly, keep the 'reward' as connected to the good deed done as much as possible. As in my first example, the quick work cleaning up the bedroom meant we had more 'time', so I made a point of using that time in a way which was desirable to my son. Doing a great job keeping toys picked up might mean having a moment to trade out toys they've lost interest in for other toys in storage. (we rotate toys out regularly, so letting them 'go shopping' in the storage is fun). Or at the grocery store, cooperation and staying close could warrant a spontaneous "You know, I've had such good help from you today, you've been making good choices-- you can pick out a fruit/vegetable for your snack today." Spontaneous, unexpected rewards leave lasting memories and don't set us up for 'if you do X, you'll get Y' and all the negotiation and whining that often goes with it.
Good luck!