If mine is really in a bad condition then I use sticky stuff remover to remove any last bits of glue and dirt from my mat before spraying with the repro. I dont always do this, because sometimes the mats arent that bad. On mats that I am doing for the first time since buying new, they usually are ok, but re-newing them several times does start to leave a build up of old glue that needs removing.

THEN

Sometimes after re-newing them, the glue is too sticky and it becomes impossible to remove the new cut shapes from your mat without tearing them. To prevent this you need to remove some of the 'stickyness' by seasoning the mat. It seems there is some debate on how to do this, I watched a demo by someome selling the cricuts at a craft show and they recommended using your hands simply because the oil from your hands does take off the stickyness and season the mats, however you can also do it by using some scrap paper and just putting it on, lifting it off a few times or using a clean cotton cloth to dab it with before the first use through your cutter.