Umm, I usually just make sure that they are clean, which I admit can take some effort when there's mud, leather fat and engin grease under them. But I think it looks disgusting when people have a long black line at the end of their nails.
I do have nailpolish on one of my finger nails, since it cracked near the top when I was trying to change the stir-ups on my sadle, so I borrowed some of my mums clear nail polish to try to keep the nail together, it still hurt like hell though.