I buy a big roll of tickets at the beginning of the year. They earn tickets for completing their school day with a good attitude, completing certain projects, completing a book, encouraging each other, etc. They can also lose tickets if they are choosing to be snarky or lazy.
I have a treasure box full of items that they can purchase with their tickets. I try to keep the items useful and fun (colored pencils, journals, books, stationery, fun toothbrush, colorful hair rubberbands and clips, etc.)
I would love for my girls to just do their school work joyfully, with the only reward being thier satisfaction with a job well done. ;) We just aren't there yet. I'm okay with offering rewards in these early years.