I am in a PhD program in Psychology. I went to a large school (UC Berkeley), and let's just say...a lot of the things TheSack mentioned did not happen for me. Nonetheless, I got into a few PhD programs at UCs straight out of undergrad.

Things I did NOT do:

-Get to know profs, sit up front, ask them questions after class, stop by their office, etc.

-hold a poster session.

-get a minor in a related area. I got a minor in an unrelated area though (Asian American Studies).

-take grad courses as an undergrad.

-start studying for the GRE at the start of college. This recommendation is a bit ridiculous. You do not need 3 years to prepare for the GRE. I bought 2 GRE books (1 for the general test, 1 for the subject test), and studied for 1.5 months for the general test and 1 month for the subject test. Scores: 800 Math, 680 Verbal, 800 Psychology.

Things I did do:

-get research experience...in two labs. This is definitely recommended. For us, it was very difficult to get one-on-one time with profs. I interacted mostly with (1) a grad student (2) a post-doc (3) a visiting scholar. All were very supportive from the beginning, when I just started research, to the the "end," when I needed recommendations. Besides the rec aspect, the process itself is very important. Who knows, maybe you will change your mind about a PhD after it. Research can be frustrating at times, but the perk for me is in the type of thinking you get to do.

-join a student organization. I joined a few psychology student groups and Psi Chi. I am not sure what the benefits of these were. Psi Chi just reinforces your high GPA.

-get a high GPA. This can be a screening factor, I hear.

-write an honors thesis.

-figure out what area you are interested in. If you want to be a clinical psychologist, you will have to take a couple years off after undergrad to volunteer/work in clinical settings.

Other tips:

I would recommend seeking projects that are very interesting to you as an undergrad. I worked on a couple things that I was not that interested in continuing with in the future; I did not realize how this would affect me beyond undergrad. Basically, you need to write about your research in detail, and schools like it if you are a "good match." At interviews too, they will ask you about say, your honors thesis, past work, etc. It's a lot harder to convince someone of your interest in one subject if you have no prior experience in it.

Not saying that TheSack's recommendations are not good, but that there are many ways to go about this (getting into a PhD program). I am a bit on the shy side, so getting to know famous profs was not really viable for me.