Plain old Crisco shortning on the rail. Scent free, can find it at any grocery store and for like $3 you get a tub of it, slick as snot. I use Bohning string wax on my string, and with this combo I have over 1,000 shots on my string and the serving looks like new. I keep it in a little No.11 BP cap tin in my hunting pack.

I also use this Crisco in my blackpowder pistol to cap off each ball in the cylinder.