Use "scuff kote" or "scuff guard" shoe polish type stuff in an aplicator bottle (those with little sponges). Must be the Scuff stuff not just shoe polish (not all places have it, I got mine from sainsburys, asda didn't have any) and get a reasonable brand, only about £2
Trust me, 2 coats of this stuff lasts for months and months! Easy to apply and no mess!