Catching up on my reading this morning I found a quick introduction to using your directional scanner in WH space over at K162space. If you are venturing into wormholes then this is one of the most imortant things to get right. Keep in mind that the whole post is referring to using your directional scanner at maximum range and at 360 degrees. This gives you the best chance to see scan probes that have been launched to find you. When missioning (or running plexes) in lo-sec or 0.0 a watchful eye on the d-scanner could also help you avoid any unknown pilots you see in local. If you see these pilots drop combat probes you will probably want to make ready for your escape.
For those of you simply using your directional scanners in PvP combat where you are trying to determine where your target just warped off to (for instance) there are some slight differences. The key difference here is that you will want to narrow the beam so you don't get every ship in the system. When doing this it is important to keep in mind that the scanner is based off of your camera view and not the direction of your ship. That means it is very important to align your camera properly if you hope to sniff out that scum you are hoping to kill. This can take a bit of practice and a little calm in the heat of battle.