Some people call this concept "Pay it Forward". I guess that is supposed to have a warm fuzzy ring to it. To me, when I hear 'pay it forward', it makes me think of owing something. Like I have to do something good for someone else in order to settle some debt I'm not even aware I have.
I like the phrase "Help Someone" much better. I don't owe anything. A good deed isn't something I do because I'm trying to make up for something bad I did (or might do in the future). A good deed is something I do because, well, I want to. Because I desire to be of aid to someone else. Not because I have to, or someone told me I should.
When I am grocery shopping, and I help another shopper find an item on the shelves, I'm doing it because I see that person having a need I can fulfill. Same thing when I'm shopping and I ask if I can reach for something on a shelf that is above the height of a little old lady (or sometimes a little old man), or a person in one of those motorized shopping carts. Or when I see a frazzled mama with tired kids in tow heading to the same checkout lane as myself and I tell her to please go in front of me, even though her cart is much fuller than mine. She obviously would appreciate getting out of the store all that much sooner than if I were to race in front of her so I didn't have to wait for her to empty her cart and pay for her groceries. I do it because I would want someone to extend the same kindness to me if I were in the same predicament (searching but not finding, trying to reach but just can't, trying to get home with my shopping-weary offspring, etc).
It's the old golden rule: Do unto others as you would have done unto you.
There are many ways we can be of help to others, not just the few grocery store examples above. I'm not going to list more, because I'm sure you can think of plenty on your own. Just look around you, think of people you know, places you go, and ideas will come to you.
Call it 'pay it forward' if you must, but for this week, I challenge you to help someone in some way.