I always find that missionary position is the one that makes me sore, it's a combination of their weight on me and the fact the angle isn't always perfect and rubs the top of the inside of me and makes it sore. The best for not getting sore, for me, is me on top and you can vary that by doing reverse cowgirl. Also I love doggy and find that rarely makes me sore although everyone's different of course.
If you find yourself getting sore just be more specific on the positions you prefer.
Back on topic, this week has been God awful for me, however last week was my best ever. I made almost the same profit last week as I made in the whole of the first 3 weeks of August. July was good for me too. I don't see a pattern and I don't think there really is one, I just tend to keep going and *try* not to get stressed!