It helps if you have friends that do the other jobs, I always try to do a bit of each role when I'm playing an MMO. Avoid words that make it sound like you're blaming someone. Ask questions, or provide something as a tip.

For example, while playing my scholar, I was helping out a guildy do a lower level instance, and noticed that the arcanist in the group was always running out of MP. So I just piped in "Hey --, are you using your Aetherflow often? If you use it on CD and make use of your Energy Drain you'll have a lot more MP to kill things with"

No blaming, no calling them stupid, it's a shame with PS3 you'll have trouble typing, but the fuller the sentence the less offended someone will be. The hardest part is to not sound elitist or like you're telling them how to play. Make it sound like a suggestion not an order. And always tell why. I'm sure if I said "Hey Stupid use your f'ing Aetherflow" I wouldn't have gotten the "Oh thanks, I keep forgetting about it" response.