I was reading my current issue of Reptiles and came across a great tip for those that feed in their pre killed or frozen in the cage on a substrate like aspen.
Put plastic screen mesh over one area that you intend to feed on, this will prevent the snake from ingesting the substrate. You can pic them up at your local craft store, they are used for cross-stiching. they come is different sizes and are usually less than a dollar. Another upside is because its plastic it does not rot and is easy to wash.
I know i am goin to give it a try.
Derek