I use Neatsfoot Oil, same stuff you use on leather. It is especially good at bringing an old, neglected handle back to the original luster and elasticity. When I bought a few older blades a while back, some of them had those sweetly carved, old style horn handles. Sadly, they had begun to dry out because the original owner apparently didn't know they had to do anything to them. He had taken great care of the old wood handles though.
What I did was first I wiped the handles down, really slather them down with the oil for a while. Then, once they had stopped soaking it up much, I wrapped the handles with a rag that was soaking wet with Neatsfoot. I left this in place overnight and in the morning I wiped off the excess and that was that. I maintain them by putting some more on them once/year