I haven't been to Ghana, but I've done aid work in Kenya, and was almost married to a man from Senegal at one point in my life.

I'd recommend wearing clothing that is much more conservative than what we're used to - I'm no prude, but in many places in Africa, this actually makes a big difference on how you relate to and connect with the community. Dress down, honestly, no short skirts or even bright new tennis shoes. The more of a chameleon you can try to be, the better your experience with people will be. I went to thrift stores and bought old things and upon returning, left most of it there with people who needed it.

You may check with Global Volunteer Network (an org from NZ). I've heard good things about them, they still have a price, but at least the programs have some infrastructure and support if you need it. Also, from time to time idealist.org has some interesting posts for positions in the developing world.

Anyways...hope that helps, and have a wonderful trip!