Some signs that repotting should occur include your plant growing over the sides of the pot such as in a Cattleya, getting a fair amount of slit when watering a plant potted in bark, bark that looks like it is turning into regular potting soil or moss that holds to much water or has a sour/foul smell. I'm sure there are others that I'm just not thinking of ATM.
Repotting should always be done with new unless you are using an inert media such as Hydroton, in which case you would want to clean before reuse.
Hopefully others will give you additional info as well.
Judi