Today's digital cameras are capable of producing exceptional photographic images, however there are often cases where an image can still be improved.

Digital filters by their nature do not improve images, but in the right hands, they can correct certain problems that can keep pictures from looking their best.

In the above image for example, you can see that there was some sunlight washing into the camera lens that is making our picture look kind of dull.  It would have been best to not have captured this image under those conditions, however, that was not possible.

Our first line of defense in this case is to use the 'Adjust value' filter to correct the problem.  You can adjust the contrast of the image here, or you can alternately adjust the brightness and value separately.  You can see here that stretching out the histogram to the edges makes the blacks blacker, and the whites whiter.  Be careful not to clip off too much.  What you see on a monitor may not be exactly what you see if you print your picture, and you don't want to overdo it.

All cameras produce some level of noise in images.  It is simply not possible to reproduce an image in all its glorious detail.

Low light conditions often create grainy images.  Work in better light whenever possible.  Use longer exposures with a tripod, if possible.

Low light conditions often create a lot of noise, particularly in the blue channel.  Camera circuitry is less sensitive to light in the blue spectrum.

Highly detailed pictures can often produce odd artifacts, such as black dots or colored patterns.  Additionally, most cameras save pictures in Jpeg format, which is a 'lossy' format.  This means that more errors will be introduced into the picture in order to save space in your camera's memory chip.

A median filter, under the Convolve menu will remove isolated pixels like the little black dots, and reduce grain.  The median filter can often remove too many artifacts though, causing a loss of detail.  You can use the 'Fade last action' item under the filters menu to reduce the loss of detail after median filters.  Fading the action by around half usually produces ideal results.

Since cameras often produce very large images, if is acceptable to loose a little bit of detail in needed to remove enough noise, assuming you will be scaling your picture to a more viewable size.

Digital cameras produce very large images, often too large to view all at once on a monitor.  For this reason you will have to scale your image, and this process by necessity reduces the amount of detail in your picture.  Don't worry, this can actually be a good thing to a certain degree.  You see, you are also reducing that noise and other artifacts that your camera's CCD chip and compression settings has introduced to your image.

Use the Resample option under the Image menu to scale your picture.  Type in your new size in pixels.  Using the 'Constrain' option will keep your picture form becoming stretched.
Use the 'Bi-linear +' option for the best scaling.

All images really need a little bit of sharpening after being scaled.  The 'Digital camera enhance' filter under the Sharpen sub menu will give you the best results. The Digital camera enhance filters uses an unsharp mask (which sounds odd) sharpening algorithm that operates on the luminance channel of an image.  This causes the filter to sharpen detail while not enhancing the appearance of color noise at the same time.  At the same time, color noise is reduced with a box filter in the chrominance channels.
Ideal settings are almost always right in the middle.  You don't want to over sharpen, or under sharpen.
There is nothing magical about image processing filters of coarse.  The sharpen filters merely increase the impression of sharpness, and do not actually improve any detail.  Successive uses of the sharpen filters will not bring an image into clearer and clearer focus, but instead will end up giving you something that looks like a bad photocopy.

When saving, your should pick the format that works best for you.  If your are using your images for print or other work that required the best possible quality, you should save your picture in a lossless format.  Targa, Tiff, or BMP will do fine.
If you are saving for the web, then file size of often more important than image quality.  You should balance the two based on your needs.  Jpeg is almost ubiquitous for images on the web.  Jpeg quality settings range from 0-100, with 100 giving you the best quality.  Even then, some detail is lost and some artifacts will be introduced.  Gif is another popular format on the web, however it is also lossy, in that images are converted to 8 bit - a mere 256 colors, down form 16 million.  The newer Png format is better, allowing for fairly good compression with less loss of detail, and it's supported by practically all modern browsers.

Here, you can see two jpeg images with different compression settings.  The first is at a medium setting, and the second at a low setting.  you can already see the loss of detail in the first, but in the second you also begin to see small squares (called metablocks) and odd patterns.
The advantage of the higher compression is that the file is smaller.  The first file is 10k, while the second is only 4k in size.  You have to decide which is more important to you.

Before and after.  Here, saturation of colors was increased slightly to bring out the intensity of the color.  The sky and mountains in the background were still a little lackluster, so the Polarize filter was applied.  In PD pro, the polarize filter is used to increase the intensity of skies and other items in the blue range.  It is very useful when you don't have the beautiful clear blue skies when you're taking your photos.

White balancing lets you adjust the overall color shift in an image based on a white point.  This basically tells the software what part of your image contains an object that should appear white, and the rest of the image is adjusted accordingly.  Use this when your photos were taken under unusual lighting circumstances, such as incandescent or florescent light, which generally causes colors to be cast to reds or blue/greens, respectively.
Note, however, that sometimes you want your photos to have an overall color cast -- for effect.  Say you took a photo of a sunset.  You would want it to look nice and golden orange, so white balancing would be a bad idea.