Question: I've noticed a lot of my pictures look muddy. They don't have very high contrast. I know I can post process some pictures, but what is the secret to having nice contrast in a picture?![alt text](https://i.stack.imgur.com/aDZWC.jpg)

Answer: 
Please, don't try to fix it later. Another teacher (where I teach photography too) says there should be no such thing as photo-correction, only photo-enhancement. Actually, I don't believe in the latter either ;)

You have to be able to produce sharp pictures with good clarity and plenty of details straight out of the camera.

If you are not, then hundreds of things can be wrong but without an example we cannot tell you what. Some of the most common problems:

* Camera shake: You have to be above a certain shutter-speed (depending on your lens' focal-length and whether it is stabilized) or use a GOOD tripod.
* Too high ISO: Cameras can reach very high-ISO nowadays but it does not mean they should, particularly small cameras with ISO above 1600 will make everything muddy. When lowering the ISO, take care not to use a too-slow shutter-speed though.
* Noise reduction: Some cameras have noise-reduction enabled by default which causes details to show up as muddy. Among small cameras, particularly those with a lot of megapixels (10+) you generally don't even have a choice.
* Out-of-focus: Your camera may tell you that things are in focus but it can still be wrong. If you are using an old-lens on a modern camera, this can happen quite a bit. Some high-end DSLRs allow you to correct this using 'focus micro-adjustments'.
* Your camera or lens is crappy: The biggest mistake when buying a DSLR is spending all your money on the camera and then buying the cheapest lens you can find. I'd say all modern DSLRs are quite good now, but not so of all lenses. Tiny camera with too many megapixels.
Another cause might be your lens. Long zooms, with their large number of elements and multiple surfaces can significantly degrade contrast and colour. In my case when I moved from an 18-125mm lens to a high quality 17-70mm lens the images had significantly better contrast and better colours. It didn't hurt either that they were so much sharper.