I'd like to add a couple more points to the helpful ones mentioned above.
First one is surface tension. If you can alter the surface tension of the place of impact, it is definitely going to make a difference in the intensity of the impact. For example, wet surface spanking. If your ass or the object is wet, it's gonna be more painful. So will be the case in any other body part.
Second one is reducing wind resistance. Some paddles are designed with holes in them to reduce wind resistance, making the swing easier and harder with same level of effort. Even though this has it's drawbacks, this is a very good example of 'improving the intensity of impact play'
Finally, a bit of creativity. You could always play with the surface of the toy. Attach or connect different things to make it more rough or abrasive or etc. But do these with enough knowledge of the effects and the methods and ways of using.
Also, having a good grip on the toy is also important for me personally. If you are using a heavy toy, this would help you big time. Taping the handle was a common method used to improve paddling before, which I've found very useful.
Devil