That's right watch out for leopards in your pictures. What am I talking about? Splotchy and uneven lighting can cause the subject of your photograph to appear spotted. Other lighting problems that might make a photograph unsuccessful are harsh light and shadows, improper white balance, and unnecessary flash.
First, let's tackle how to avoid those pesky leopards. Trees are usually the culprits of this effect. Whether a tree is casting a shadow on the subject, or light shining through the leaves in spots, this effect will occur. A tree is a nice, cool, shady place to position your subject underneath, but on a sunny day you will have problems with the light shining through the leaves. Using a large diffuser you can block that light from hitting your subject but that usually requires another set of hands. The simplest solution is to wait for a cloud to pass over the sun, or move to another location with more even lighting.
A big misconception in photography is that a bright day and cloud-free sky will make for great photographs. The opposite is true. Just like you are more likely to get sunburned on an overcast day, you're also more likely to get better pictures. An overcast sky prevents hard shadows, allowing you to photograph in any area you like, and also not worry about distracting shadows on a person's face like from the nose, eyes, chin, etc. Without a bright sun glaring down, you also won't see squinty eyes in the picture.
With digital photography we've begun to understand white balance a little better. When all we had was film, you had to buy special tungsten film or use a myriad of filters to correct fluorescent and incandescent light. Now, a digital camera can do it all for you. In most cases, setting it on auto white balance will work. However, you may notice a green tint when shooting under fluorescents, or images that are too orange when shooting indoors, or blue when in shadow. Experiment with the different white balance settings on your camera. Usually they are clear icons, a traditional light bulb is for tungsten light the warm bulbs usually used in your home. A rectangular light bulb shape is fluorescent lighting. Other options might be a sun or cloud. Those are pretty self-explanatory. Another option you might have is little house with a shadow next to it, this will warm up the picture if you are shooting in heavy shade. If you want to get really technical on your D-SLR, there is actually a product called the ExpoDisc. It looks like a filter and you simply hold it over the lens in whatever lighting condition you will be shooting and take a picture. It will turn out gray, and then you can use that frame to set a custom white balance. If the lighting changes, just do it again.
Finally, it is important to know when to use flash, and how to control it. If you do have to photograph outside in the sun, sometimes it helps to use flash to fill in some of the shadows. When indoors, or at night, try using the night setting on your camera. This is usually an icon with the moon and stars, or a person in a black box with a star above their head. This uses what is called slow-sync flash. The camera takes a picture long enough to let in the ambient light, while the flash goes off to freeze the subject. If it is daylight and you are taking pictures inside, try to look for nice window lighting. This is usually soft and subtle lighting that can create a beautiful effect on the object or person you are photographing. In this case, make sure to turn the flash off. One last note on flash, you can make it less harsh by covering or bouncing the flash. For D-SLRs that use a hot-shoe flash, you can point it straight up and let the light bounce off the ceiling, or put a diffuser or soft-box over the flash for more spread-out light. Even simply covering the flash with a little wax paper will probably make a big difference.
By paying attention to the lighting conditions and knowing how to use all the functions of your camera, you can get much better pictures than just blindly shooting.