Refuse what you don't need
This runs the gamut from second helpings of dessert to iPhone upgrades.
At the risk of sounding preachy, I think we all need to take inventory of what we need as opposed to what we want.
A couple of quotes from my extensive collection:
We have to love what we have to love. (anonymous in that I have been unable to find an author)
I have learned to seek my happiness by limiting my desires, rather than in attempting to satisfy them John Stuart Mill.
Of all the set books we had at school, one of the few passages I remember is from Mill's "On Liberty" where he said that you are free to do whatever you like - as long as it doesn't impact other people. Taking/using more than you need definitely impacts other people. Think about it.