Knowledge about producing all types of media: music, images, videos, etc...
Always strive for quality, purity, and simplicity.
When sourcing media to include in a project, go the extra mile to get that higher-resolution image, or find that song in a lossless format. It will pay off when you have gorgeous finished product. Each corner you cut will add up.
While producing media, don't do things to it that degrade the quality unnecessarily. Often during editing a "generational loss" can occur at each step in the process. An extreme example would be capturing a video by holding a camcorder up to a TV screen when one has access to any other video source.
A more common mistake is using a lossy file format for intermediate files in a project. Use lossless formats for intermediate files whenever possible. Far too often the same data ends up being subjected to lossy encoding many times over.
For example, say you have a collection of MP3s which you are going to burn to an audio cd, and along comes a file in WAV format. Just to make things neat and tidy you convert this file to an MP3 as well. DON'T DO THIS. Leave the file as a WAV, or convert it to a FLAC file if you want to save space.
Another example: While making a kick-ass desktop wallpaper, you download a picture to include in it. Before including the downloaded picture, you decide to crop it, and save it as a JPEG. DON'T DO THIS. If you make changes to a picture that will be used in a larger project, save it in a lossless format like PNG.
While the previous tip about generational losses is arguably a sub-tip of this one, I would like to stress the importance of not doing strange things to your data during editing. Pay attention to and avoid changes to things like frame rate, or sample rate, while editing media.
The final product should be the sum of its constituent media files with only the bytes changed that the artist (you) intended to change. Avoid unnecessary and often subtle mutilations of your data.
In today's chaotic digital world, a hunk of data with no descriptive information attached is nearly useless. Tag files with all the metadata you can. A good example is album art attached to music files, this makes them much more attractive. A good command-line MP3 tag swiss-army-knife is eyeD3, and a very powerful GUI MP3 tagger with easy batch processing is EasyTAG.
Publish your media in the most widely usable standard formats. Don't trust that a particular tool spits out files that are industry-standard. Do a bit of research first and its easy to produce professional quality media.
Strongly consider providing your media in a lossless format, perhaps as a secondary download option.