I gave mine a good scrub to get all the grime off it and used the Auto Glym it gives it a nice shine and a waxy feel to it that hopefully keeps the dirt out the grain .Auto Glym do a vinyl and rubber care spray product which simply wipes on. I use it on mine and it's commonly available from Halfords and the like and seems to produce good results.
There are plenty of folk who use boot polish for a similar effect. I'm sure there are countless other suggestions too