I always start with this great book. My third graders love it, and we have lots of good discussion about the difference between bucket filling and bucket dipping.
I find that when we get started, it helps to give the students a little note to use. Somehow, just a little bucket graphic seems to get them on the right track! Once they get more comfortable with writing positive comments to their classmates they usually just use their own paper. Here is the note I've used the last couple years -- click to download it for yourself.
After we have made filling buckets part of our daily lives, I like to read this book with the students: