Just do a few full throttle runs on the highway. When the preconditioning thing shows up on my screen I'm usually pretty close and it wont actually do much to heat the battery by the time i plug in.
So i will floor it to about 95mph then allow the regen to get me down to about 65 then do it again. Doing about 3 pulls like this will heat up the pack. If your pack is really cold you may have to do a few more but this works very effectively.