I'm applying right out of undergrad to bio anth programs, and I seem to have done okay with acceptances. (I'm in three places - waiting to hear from a few more!) Obviously, I'm not sure exactly what helped me get in, but here's some advice that numerous undergrad advisors and research advisors told me that I used in my applications.
1. Grad programs are looking for people who are capable of doing independent research. If you have research experience, I've been told that it really helps to emphasize it in your SOP, and to connect that research to the project you're hoping to work on. It also helps to emphasize independent roles you had while doing research, whether it was designing your own experiment or organizing other students.
2. Make your SOP as focused as possible. When you're talking about research interests, make it specific, frame it in the context of current theories/debates in the field, and use details. A lot of people talk really broadly in this portion of their application, and I've been told that a lot of schools are looking for something very specific. It also helps to frame your SOP to the particular school you're looking at; I mentioned particular resources at each school that I thought would make it a great fit for the research I wanted to do and talked about specific people I thought that I could work well with. You're arguing to a committee not just that you want to go to grad school, but that you want to go to grad school there - out of all the places you could go to study, say, modern human variation, why is University X the best fit?
Obviously, there are people here with a lot more experience than me, but maybe this will help as a jumping off point!