If you don't have any cable liner handy, a cotton swab with hollow plastic stem will do. That's what I use on my bikes after my last bit of cable liner ran out. Snip off the cotton ball ends. The stem is hollow and just the right size for most shifter and brake cables. I use 'em on my bottom brackets and the convoluted metal guide noodles brazed onto the seat cluster on my old Univega with cantilever brakes.

Works great. Reduces cable friction a little too.