Always keep the lighting in mind, never take a photo into the sun or against a window, if this can't be avoided be sure that your flash is on to compensate for shadows.
Before taking the photo, not only should you make sure your subject is where you want it, but look at the other things in the shot and try to avoid distractions at the edges of your photo. For example: Someone elses shadow or an object like a rubbish bin or litter.
When taking portraits, be sure that your subjects are well lit, remember squinty eyes don't make for flatering photos. Try to take the photos in the shade.
For those who have the option, when taking pictures of people, put your camera into "portrait mode" you can ussually find it by an icon in your menu that looks like a womens head and face.
When taking photos of landscapes or scenery, and you have a lot of distance to cover that you want in focus, switch the settings in your camera to "landscape mode" usually found by an icon that looks like mountain peaks.
Finally the obvious, always use a decent camera bought from Jessops