Show up about 5 minutes before their first meet in MK (they usually meet at 9:30, so be there at 9:25). You will be the first, or very close to the first, person in line.
Same advice in Epcot - being there a few minutes before their first M&G of the day is your best bet for short waits. Their Epcot que is indoors - so I'm pretty sure it's air conditioned if you do get stuck there for a bit.