Agents also like volume business. If you tell an agent you will shoot 5+ scenes, you are more likely to get them to work with you.
Another thing is time. If you can shoot quickly and promise a model she'll be in and out with no waiting around, you are more likely to get a better rate.
Request new performers, too. Stars will often feel like they deserve their rate, which they do. Stick with newbies who are on their way up. You might catch the next Sasha Grey. I know this from experience.