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: American Pale Pacific Gem SMaSH
Recent Meads: Mead Day '11 Ginger Metheglin, Mead Day '12 Traditional (orange blossom) Mead, Simple Cyser '12
Primary: Red X Weimea SMaSH
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider, Simple Cyser
Planned: Hop Metheglin #3 (NZ hops), Trad. Gesho T'ej