Here are my suggestions:
1. I write this in every post, but the most important thing is to model good behavior. Model the Golden Rule (which is based on Matthew 7:12, Mark 12:31, and Luke 10:27): Treat others as you want to be treated. In the checkout line, would you let someone go in front of you because he has only a few items? Do you give money and things to charity in the presence of your kids? Extend lots of grace and be willing to let someone else – spouse, kids, friends, stranger—be first.
2. Almost as important as modeling selfless behavior is talking about it. Saying things like “Since you’re the guest, you can choose first,” or “Let’s take turns. You can choose what we’ll play and then I’ll choose.” reinforces the behavior that your kids are seeing. 
3. Read books that model sharing and selflessness, such as You Go First, which is part of the Little Critter series.
4. Kids who feel confident, secure, and loved share much more readily than kids who are missing one of those pieces. Work on making sure your kids feel confident, secure, and loved in general, and those feelings will spill over into their interactions with each other. One big way to do this is by spending 10-15 minutes of one-on-one time every day with every kid. It might sound like a lot, but having time when they don’t have to share your attention with anyone (including your phone, spouse, or the tv) will make all the difference in the world.
5. When you can, employ the “I’ll sit with you while you wait” technique. When one kid has a coveted toy, play with her sibling until it’s her turn. The sibling with the toy gets uninterrupted time without worry that her sister is going to swoop in and steal it, but the sibling who’s toy-less gets an even better prize – devoted time with mom.
6. Catch them being good. Lavish your kids with praise when they’re sharing nicely. They’ll remember it.
7. Know when to let them work it out. Your kids are going to squabble over toys and turns. It’s going to happen. Know when to ignore it (within reason) and let them work it out on their own. If you always intervene, they’re going to expect you to continue to intervene.
How do you encourage your kids to share with one another? How do you praise them when you see them being selfless?
