I don't get the pre-trip depression so much. I don't start to get excited until about a month or so before (FYI - I am not the "planner" in our relationship). I get so stoked the week before we leave that I'm counting down the days and hours. That will be the week after next for us. Right now I'm arranging all the little niggly emergency & how-to's procedures for while I'm gone (I'm the IT director at my architectural firm), and making the last few construction observation trips for the projects I'm responsible for. This week and next I'll be pulling 10-15 hours per week more than I usually work (my normal work week is 45-50 hours) so I'm usually too tired to get really stoked too far beforehand.
What hits me bad is the post-trip depression - and that doesn't really hit me until I've been home for 2-3 days and have gotten settled back into my routine life - after unpacking, doing the trip laundry, kids have put away souvenirs, KAMommy's done with the photo album, we've been back to work (& school for the kids) a day or two, etc.