How can I ensure that DVD's I burn on my Mac will play on regular DVD players?

I've burned several DVD's (using -R format) of family movies, and have only had limited success actually having them play in regular standalone DVD players. Why is this? It is so frustrating!!! I've used only DVD-R's, and I've heard that perhaps I should try +R. Would this help?

I'm using a Mac G5 and have used iMovie and iDVD to do the producing/burning. Please help.