Any person can bring about great change for others and them self. A gesture can be the deciding factor in the bringing about of great changes. When faced with the opportunity to give that to others make short order in participating.

We have that never ending competition in our society that conditions to bring about the choices of opportunity.
When faced with these choices, give freely to others and be a karma farmer.
Yes you can build up good karma simply by being open to the idea and take that second as that is usually all it takes.
When you notice, and we do even if unconsciously, that others around us send that signal.
Give acknowledgment to
Well done tasks
Unconditional giving that is not necessary
Changes within them self that bring them prosperity
What appear to be little or trivial additions to your relationships.

There are hundreds of instances of this opportunity, as i said, if your open to looking for them.
The result is much lifting and strengthening of any relationship and does bring good karma.
One of the quotes we have here on the site says it quite nicely and I would like to quote it. :)

You cannot bring about prosperity by discouraging thrift.
You cannot strengthen the weak by weakening the strong.
You cannot help the wage earner by pulling down the wage payer.
You cannot further the brotherhood of man by encouraging class hatred.
You cannot keep out of trouble by spending more than you earn.
You cannot build character and courage by taking away man's initiative and independence.
You cannot help men permanently by doing what they could and should do for themselves.
-- Abraham Lincoln

Allow others to their right to self determination without prejudiced involvement.

Lift those around you and together they will lift you.
It only takes a second.