Maybe the process of running through a mental checklist before departing is helpful?
Plan where you're going, what you you need to bring with you to accomplish whatever you intend to do when you get there, and the route you're going to take, then forget about all of it. Walk around the bike, checking whatever you need to, including lights. Turn off the key but leave it in the ignition. While suiting up, review what each piece of gear does to make the ride more pleasant or safer, visualizing riding while wearing it. By the time you're fully suited up you have probably reviewed the entire route, or if it's just a Sunday morning ride formulated an idea not only of the roads you're likely to encounter, but also the "feel" of the ride. By the time you start the engine you have already ridden for an hour in your mind, even though it took only a couple minutes to suit up.
My centering exercise, developed over years and honed to a fine edge by endurance racing, where you have to be right on top of your game from the moment you leave your pit with hot tires and a full tank.
The longer it's been since you last rode the more important this exercise is, as it must include visualizing the bike's feel as well as your riding it. As I said before, focus is all important. This is easier when the weather is not perfect as it takes longer to suit up. On a beautiful summer day with the temperature in the low 80's and roads that haven't changed condition for months it's harder to do, but just as necessary. Actually, it can be more important as it's much easier to get complacent.
It's been 2 years since I've taken the MSF Experienced Rider Course and I'll probably take it again this year. Great class.
Actually, I take an Advanced Rider Course every time I get on the bike. The instructor is a real bastard -- critical of every move, notices every tiny mistake, and makes me aware of it right away in no uncertain terms.