This trick has probably been posted already, but I discovered something a couple days ago.
I mixed in some Cremo cream with some latherable shave cream (arko) and lathered it up with my brush. Holy cow it was super slick. Even after I had shaved and the lather was gone I still had a super slick film left behind and I could do touch-ups without applying more lather.
I decided to try the same thing with a shave soap this morning and got the same results. So for those of you that use a shave brush and shave soap or cream, try adding some Cremo, it's awesome.