Lighting .. that is always something that we need to take into consideration when taking pictures. Alot of people think that they have to use their flash to get the best results. Maybe focus something along the lines of natural lighting.
Posing .. candid shots work out nice. Don't always use posed shots.
Cropping/Angle .. try for different angles when taking pictures. DOn't just take a picture that is straight on.
Less background or clutter ... I see so many wonderful pictures where the photographer hasn't taken into consideration of the background "clutter". More is always less.
I will think of more for you ... this is things that I always try to remember while doing a shoot