Obviously one of the most important activities when planning for a trip is to pack the right items. In an effort to reduce the amount of luggage I needed to bring on my last trip to Europe (went to Italy's Almalfi Coast, Greece's islands Mykonos and Santorini, as well as Istanbul, Turkey), I didn't pack everything I needed.
Not having a long sleeved shirt or a jacket proved to be a big problem and a pair of black sox and a pair of black shoes would have been nice (I should have left my brown shoes at home).
Previously when I traveled, I would pull out my last packing list and make small modifications to it and then mark down each item as I finally put it in the suitcase or my backpack. This way obvious things like your cell phone travel charger, digital camera/charger, ipod stuff, sunglasses, sunscreen, etc., all end up in your bag.
I was by keeping these lists and noticing how much clean clothes that I had at the end of the trip that I got the smart idea of trying to minimize the amount of stuff I would carry with my to Europe. Whereas my last trip to asia I packed one of the large size suitcases, this time I packed a bag that could be carried on the plane (though I checked bags anyways, so it didn't really help me travel more easily anyways).
Anyways, my tip is to make a template for various kinds of trips and then use that as a start for your next trip so that you don't forget something important, like a jacket. I think I will require at least four templates: business trip, summer/beach trip, ski trip, and family trip (visiting my mom or brother).
I'll post some templates later.