Using a release agent to cast your parts will prolong the mold life, but you may not want to do that if you are painting or applying some other post mold finish to your cast part. Releases can prevent good paint adhesion and/or interfere with your finishing process.
As the mold is used it will start to dry out and become brittle. This is because every casting takes a little bit of fluid from the rubber. You can slow down this process by treating the mold with mold dressing (for polyurethane rubber) or silicone fluid (for silicone rubber). Apply the treatment at the end of a day or week and let it sit in the mold until the next use. Wipe out any residue before you begin casting again.
Keep in mind that every situation is unique - please consult Polytek Technical Support with any questions about your molds.