ATSKO wash and re-proofer (make sure it's the spray bottle and not the aerosol one) .. you can get it in the hunting section of Walmart (found in Bellingham though) or 3 Vets near Cambie and Broadway had it last year.
It's dirt cheap. The wash is good for down and anything you don't want to have fragrance (bear country, hunting etc). NikWax works too, bit pricier, but you can find all at Mountain Equipment Co-op etc. Though keep in mind, you also have to wash it really well first.
You basically have to wash the jacket first with something that has zero brighteners, additives, fragrance etc. This wash for "performance" fabric doesn't contain anything that will interfere with the vapour releasing properties found in the membrane of your waterproof breathable fabric. Then spray it with re-proofer, let it hang dry, then put it into the dryer at as high of heat as the jacket can tolerate to activate the water wicking properties of the re-proofer.
I'm assuming it's a waterproof breathable product (ie. goretex, eVent or a version thereof).
Apologize if you know this already, but the idea behind WPB fabrics is such:
the pores in the membrane are so small that water from the outside doesn't get in, but water vapour from your sweat etc gets out. A lot of times, the pores get gunked up with dirt, sweat etc, so vapour can't get out and wets out from the inside and people think it's actually soaking from the outside rain. Even if water isn't beading off like new, a lot of WPB jackets rarely wet out from the outside.
Hope this helps.
Actually, try washing it with an appropriate detergent and then putting it into the dryer at as high heat as you dare. It's the heat that re-activates the water-wicking feature and clear out the pores.. thereby making it waterproof again. If it's a high quality WPB jacket, that should be all that needs to be done.