How to Avoid Breast Implant Extrusion? What are the factors that contribute?

In some rare situations (aside from irradiated breasts), the skin can thin out and there is very little tissue to support a large implant.  Changing to a smaller implant is an option, as is using Alloderm to add support

The two biggest factors would be infection and big implants with thin tissue coverage.  The best way to avoid it is to follow your Plastic surgeon's advise regarding implant size.