Acting nice for karma's sake? If I recall correctly, karma is constituted of two parts - action and intention. Which means if one wishes to "gain" positive karma, one would have to conduct a positive action AND do so with a positive mind. With that being said, if I were to help an old lady to cross the street for the sole purpose of obtaining good karma, am I going to get good karma?
This is a triple win in term of accumulating and strengthening the effectiveness of the karmic action:


Being nice
Knowing being nice has consequences
Acting knowing there are consequences


When doing good, believing in Karma and knowing there are consequences, reinforce the good results.