Well I agree with Cinder's that this question is rather broad. I would say, 1.Religion, it was once mainly Judeo/Christian values. Now we have had an influx of other religions that have made slight changes to accepted culture. 2. Philosophies taught at higher education. It influences laws and politics. 3. The change in moral values, including sexual issues and moral relativeness (absense of absolutes) 4. Science, unfortunately science is often not a pure practice and can be a tool used for political of phylosophical agendas. 5. Immigration, It used to be that people came to the US to become Americans. The trend now is to bring thier culture and language with them. Instead of adjusting to American society, we are becoming ever more pluralistic. Last but not least would be the Mass Media.
What do you mean by influences? What do you mean by culture? Or, more importantly, what do you mean by American culture?
Everything influences culture -- literally. All the way from the religion you practice (or don't practice) to the food you eat. And, much like with every country, there is no singular culture. The culture of California is different than the culture of Maine. And the culture of Northern California is different than the culture of Southern California.
There's no way to give any kind of definitive answer to that question. It's too broad and too subjective.