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: 
Shoot with good/interesting light! 

See the image below, the reason (ultimately) that the image has nice contrast is the original image has contrast on her face, e.g. once side is quite bright, and the other quite dark, because there is a window beside her.

Try: putting your subject inside, with the lights off, facing you parallel to a window, so the light from the window hits the side of their face.

Also, I post processed this photo, adjusted the exposure (probably down), adjusted black and white levels etc.

![alt text](https://i.stack.imgur.com/MmvSF.jpg)

Here's a go at improving the image's contrast in post. Note:

* It'd probably be easier to do this starting with the original image
* We're starting in a difficult spot, the shirt is very bright and the face has very little contrast on it

Steps I took:

* Used the 'curves' tool in photoshop to increase contrast every where except the shirt
* Used the dodge tool to lighten up the eyes (I'm not sure I did a great job of it)
* Made it black and white
* Added some overall contrast with UnsharpMark (see LCE in my other answer)
* Ran the smart sharpen filter at 25% at 0.8 pixels for a bit of sharpening

![alt text](https://i.stack.imgur.com/QvELt.jpg-----)
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.