When I 1st started DMing again after nearly 15 years, I just grabbed the first new DnD 4th edition module and dove right in. There's no problem in doing that, but as I went along and started progressing through more modules both the players and I started to feel like tings were getting a little stale. I didn't really have much back story, no over arcing campaign ideas that I could drop in as hints that might elude to some larger scheme than just what the module provided as "local quests" to solve.
Here's what I am talking about. First, you need a villian....someone the players either know from past dealings or have some special hatred toward. In my own personal experience, these proved to be one of the following, Strhad from Ravenloft, Tiamat, Asmodious or Lloth and maybe a few others. All of which have vexed my players in the past and would provide some instant hatred for my players.
Now all you need to do is come up with a simple, yet diabolical scheme on why that villain is screwing with the players. Something involving gaining more power and using said power to devastate countless lives always is a good motivator for your players, especially if any of them have a decent "goody 2 shoes" player among them.
So far you have the who and why, now just start creating the how. How is the villain going to pull the strings? Through the use of some minions of his? Directly intervening? A combination of both? I tend to use the latter for a few reasons. 1st, it keeps the villain's identity hidden from the players which helps build some suspense. I just continue to drop some common clues that all point so the same villain (or villains). Be they symbols etched on the walls of the various dungeons and nasty places the players visit, a recurring vision/nightmare...just get creative and be as deceptive as you want. In my current campaign, there appear to be 3 rival villains that all are causing the players grief, but they don't know (and won't know until the correct time) how all of it will pan out, nor if even any of the 3 will end up as the antagonist in the end. I say this because, as much of a temptation as it is to throw out the bad guys name in the early campaign arc, try to hold off a little. Confuse your players and make them guess about who they will end up going toe to toe with in the end. Will it be a show down with Tiamat or maybe a romp through the 9th level of hell to take out Asmodeus?