You want to improve your photography? Here's a few tools every photographer should start with. Remember, fancier cameras do not make you a better photographer. The following list starts with the most important items.
Tripod. For shooting anything other than sports (and even that sometimes) nothing makes sure your images are sharp and crisp looking better than a tripod. I even use it for portraits as it allows me to get my face out from behind the camera and talk to my subjects.
Polarizing Filter. Assuming there is a screw mount on your lens to accept filters - this often helps make pictures more saturated and reduces glare.
Hand held meter. Yes, your camera has a built in meter. Bad news, that meter occasionally lies. Imagine my disgust the first time I shot in winter on a cloudy day. I had some good compositions but the pictures were all dark grey. With a hand held meter used properly - the snow would have been white. Plus, if you start to play with professional lighting, you'll definitely need one.
To use one, hold the meter so it measures the light hitting the subject. For portraits, that is usually the brightest light hitting your subjects face. However, if you're downtown (or in a canyon) and the light is coming from straight above - or if the brightest light is a rim light from behind - hold the meter at the subject's position pointing towards the camera, measuring the light which will bounce back to the camera.
If you do buy a meter, get the sales person to give you a demonstration on how to use that specific meter as there are some differences among the types.
More lenses. Get a wider angle or longer lens than you have. Once you've gotten used to the lens you bought with your camera, its always good to push yourself with new viewpoints brought on by new lenses.