How to politely say no

This is something that I struggle with every day it seems.

Whether it be to an invitation out, volunteering for an event, or just doing something around the house, I have a tendency to be a pleaser and consequently to not say "No."

This situation arose at work, where I was invited to eat lunch that had been brought in. I had already brought my lunch to work, and though I thought it was a nice gesture, I explained that I brought food from home and would be eating that. A few minutes later had to reiterate that I would not be eating lunch with the group and instead would be having my own lunch, at lunch time, that I brought in.

The hard thing was saying no and not just caving in and eating something I didn't want to eat, with people I didn't want to eat with. On top of that, I have a bunch of work to get done for them that I'm sure they would rather have done instead.