You can get stuff that you wash you ski kit in which waterproofs it or a spray on.
I do mine about every 6 months. You can buy water proofing stuff from most out door places.
I'm sure somone here will mention some brands that are better than others.
My avice. Use the Nik Wax, but use the spray on one (£7.99). The wash in one will waterproof the jacket lineing as well. Which is not good for breathable fabrics. Also wash the jacket first, but dont use normal soap powder, as this can damage the waterproof membrane on the inside of the fabric. Use the Nik Wax Tech wash (£3.99).