I always worry that being too vigorous at this stage will introduce too much oxygen, so I'd avoid something like the drill. What I do is mix up the back sweetening with a small amount of warmed water, and put this into my bottling bucket...I then rack the mead right into that. This probably mixes things fine by itself, but I then also use a sanitized skimming spoon to gently stir the whole mass up a bit. Again, I try not to make this too vigorous so as not to introduce excessive oxygen. So far I've never found any evidence that I had any incomplete mixing of either backsweetening material, priming sugar, or other additives (acid blend, etc.)
Packaged: Simple Cyser '14 (Nelson Sauvin Dry Hopped!)
Primary: Simple Cyser '15, Basic Spiced Cider, Spur of the Moment Graff
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider
Planned: Some sort of SMaSH. Otherwise, so many ideas, not enough time to brew them....