Buying a new lens

If you're not sure about which lens to buy, remember this:
for maximum versatility you want the widest aperture (f-stop), you want autofocus, and you want image stablization (or vibration compensation or something similar).

While the zoom range or prime number (for lenses that have a fixed distance) is determined by your shooting needs, the aperture will give you depth of field control, the autofocus can focus faster than most people I know, and the vibration control will give you sharper pictures when not using a tripod (like when you're on the ground chasing a small child around or at a ballgame and you need to follow the action.

Take one pic

Before you leave the house, take one picture.  If you have your digital camera set to "require" a memory card to capture, then you'll know if you are heading out with an empty card slot.  As a bonus you can check the image for white balance and other settings like metering and camera settings (aperture or shutter priority mode or manual vs. program mode)