I started my son on the neilmed sinus rinses at 2 and a half after an ENT recommended it. It was a challenge getting him to do it at first and then he got much better. Getting a sweet tart after he finishes helps! It also helps to always be gentle, even if it takes a while. Once we really forced it when he was stuffy and I think it hurt him and he refused to do it for a while after that. It's been a learning curve for him as well as us.
Just tonight I did the rinse on him and for a while it wouldn't go through. After a while of very gentle squeezes and gentle pressure, a big glob of yellow goop came out and then the rinse ran clear and easy. It's great to know that I'm getting that stuff out of there, I know it would have just sat there otherwise and caused problems later.
There was a recent thread on this topic and a mom posted a youtube video of her daughter doing it. I think that showing my son that video really helps. We also found this one of a little boy doing it.
http://www.youtube.com/watch?v=FqAYv9HZkA0