When I was in university I had a serious problem with concentration and being "in-and-out" of the zone. So for two years I made an effort to have something chewing in my mouth(started with candies, then pen, and after a couple of unfortunate blasting incidents-coffee stirring rods) whenever I was 'in the zone' or about to be.

After a while, I've been conditioned to be "in the zone" on cue by chewing on a coffee stirring rod. it works about 50~70% of the time, and I have to be not tired at all. It's good to have it invoked on cue, though. I find drinking coffee in general not helping; caffeine is great in getting hufed up for a short duration, but I try not to consume more than two cups a week.

Addendum: A sad side-effect was that now I cannot seem to have meals without being mentally engaged - be it reading or conversing.