Am normally okay during the day - it's the night time driving that gets me. Probably because I've been awake all day and knackered.
Depends on how long it is. More than three hours and I'll stop for a breather and to stretch my legs. Which is why I try to keep the journey time down.

Otherwise - lucozade sport with caffeine, sweets like haribo/skittles - anything that doesn't need to be unwrapped. Gum and mints. Switch on/off the aircon. If there's no one around, slow down to 60mph and open the window. Play some angry/pumping music and whack the bass up. This is all for a short duration and then back to normal.