Filters and Some Photoshop Action

Polarizing Filters
They help you make the sky darker. It's awesome for landscape and architecture photos (not people). It makes the sky relatively darker with respect to the ground/buildings. Here are two examples. I have a 77mm thread polarizing filter that you can borrow. (It works well on very sunny days, it doesn't do much if it's overcast).

No Filter

With Polarizing Filter rotated to maximally darken the sky

Infrared Filter
Infrared filters screw on the front of the lens and block most visible light. They allow deep red and infrared light to go through. Cheaper cameras like Rebels and D40s are actually better for infrared (compared to Canon 40D or Nikon D200/D300). Anyways, you need a tripod because the exposures are long (several seconds). Set you ISO around 400 to make it quicker.

This is what your infrared photo looks straight out the camera

Press Ctrl+1 to see the red channel; Ctrl+2 for green channel; Ctrl+3 for blue channel. You will see that most information is in the red channel. However, the other two channels hold the details (Green and Blue are much sharper than the Red one). Play around a bit with channel views. Press Ctrl+` to return to full color mode (RGB).
Now look at your histogram (go to Window>Histogram, select "All Channels View"). For this infrared photo, the histogram is shown below.

Histogram for the original infrared photo.

Note that the flat part of the histogram does not contain any useful information. We need to throw out that part. Use "Curves" to do that. Press Ctrl+M to bring up the Curves menu. You can see the RGB, Red, Green, Blue adjustment curves. In order to throw out that useless part of the histogram, you could apply the curves below.

After applying the curves, the histogram changes a bit. See the new histogram below. This expanded the useful part of the histogram and made the image stand out more.
This is how the photo looks after curve adjustment. The colors are off, but it doesn't matter since the final goal is a Black and White image anyways.

Now it's time to take this tea-color looking image and make it BW. Use Channel Mixer for this (Image>Adjustment>Channel Mixer). This specifies what proportions to use in the BW image. We'll use more red and less green and blue. Below are the settings that I used and the image that resulted from that.

Channel Mixer settings

Photo after Channel Mixer

If you are not satisfied with the contrast, you can apply a so-called "S-Curve". Bring up the curves (Ctrl+M) and make the highlights brighter and the shadows darker (see how the curve looks below). Make the "S-Curve" steeper for more contrast, but don't overkill. My curve is very gentle. See the resulting image below.

S-Curve Increases the Contrast

Image after applying the S-Curve.

That's all. If you guys are interested in learning more about color and Photoshop, and curves, and all of that, let me know. It will make you a better photographer.