Keeping fit: I like exercising, so I make it a point to do so when traveling domestically or internationally. Depending on where you are, running through the local neighborhoods is a great way to just get out of the hotel, stay fit, and see things. Walk whenever you can - I can't even express to you how much walking I did in Paris, Prague, Zurich, Frankfurt, and Seoul (and NYC, of course). I avoided cabs or transit unless I was in a time crunch or the distance was just too far to walk. Eat healthy, balancing it with getting a good sampling of the local fare. Don't feel obligated to eat every last morsel on your plate. The wait staff won't be offended! Along the same lines, don't drink too much, if you can help it
Sleep is another big one. Many people do many different things, including supplements, sleep aids, exercise (seriously), and just plain ol' sleep timing (ie, basically trying to adjust to wherever you'll be by staying up or forcing yourself to sleep, etc). Regardless, getting decent sleep is critical. I didn't have too difficult a time in Europe, but Asia killed me. I've not done multiple regions in a single trip, but many have, and there are many threads if you search about sleeping.
Keeping in touch: Make sure your phone gets service wherever you are; use Skype (or Facetime, if an apple guy); and email. If you have a VOIP service you can use over your laptop, that's another bonus.
Staying sane: I always have something to read (ipad, books not available in eformat, mags). I also load up my laptop/ipad with movies/tv shows (although, honestly, I don't always even watch them). I sightsee wherever I go. To that end, take a camera with you and take a lot of pictures. Don't be afraid to look like a tourist. It's all good, man, b/c when you're not working, you ARE a tourist!
And depending on where you're going and for how long, see if you can "drag" your SO along with you. Your room and transportation are paid for, so you'd really just be on the hook for his/her food, souvenirs, etc.
Loyalty Programs: Sign up for all the hotel programs, and pick a primary airline program. It helps to know where you're based and where you expect to travel. So, where are you based?