On an older vehicle if the seals are just dirty, even really dirty, then Meguiar's M39 Heavy Duty Vinyl Cleaner is awesome stuff. You'll still want to apply a dressing after the cleaning though.

If the rubber has already started to break down due to UV exposure, then about the only thing you can really do is replace the pieces. You can't really restore it and bring the rubber back to life, so to speak. But you can take really nasty looking stuff and make it pretty again. You just can't bring it back from the dead.