One of these things are a worthy addition to your tool box if you don't have one already. I picked this one up for A$12 at my local bike shop but I have seen plenty of them on eBay as well. I never really understood how they worked but what a breeze.
Clutch cable done in under 5 minutes.
Tighten it on to your cable, wrap a rag around as some lube escapes, give it a few squirts until lube appears out the bottom of the cable. Done.
I used to do things the hard way by taping a plastic bag to the cable filling it with oil leaving overnight to find its way through. Not anymore. A cheap as chips thing to keep cables working as they should.