If your jacket is in fact waterproof (it has goretex, or the like), and it's just that the outer repellency has worn off, you can use something like a Nikwax wash in for the jacket. You wash the jacket with a capful or more of the Nikwax, and then dry the jacket, and it sets the water repellent on the shell, so water will bead up on the outside again.
I use it on my ski shell at least once a season, and have used it on my ZR rain jacket, and ZR pants. I've bought the Nikwax at REI before.
Good luck!