Nothing. I use nothing and never have. If I ever need to take the bars off, I knock out the dust and dirt. I think if you start lubing it up you will just create a sticky mess which will make you think you need more lube. That will just make more of a mess.
My advice is to remove the throttle tube and spray brake cleaner into it to clean out all the lube or whatever else is in there (grease, antiseize, oil, dirt). Then spray down the bar end with brake cleaner and wipe it clean. Reassemble and you're done. Now, dont touch it unless you are knocking out the dirt. :D
I've been riding for 18 years and have never lubed, had a throttle stick or bind on any bike I've ever owned. My $0.03