Video compression is very important, no one wants to download a 500mb 30 second video clip. If your on windows, the easiest route to go is SUPER. http://www.erightsoft.com/SUPER.html
Once you have installed SUPER, right click anywhere on the program (yeah, I know, that's weird) and select 'add multimedia files' from the menu that pops up.
Select your video file, then set your quality. Below are the settings that I use for the "High Rez" videos you find on this site.
For compression, I use XviD. I usually set the dimensions as either 640x480 or 768x576 depending on the length and the video bitrate to 1440-2256 again depending on the length. Audio I set to 96kbps stereo.
Set the output folder by again right clicking anywhere on the application and selecting 'Output File Saving Management'.
Click 'Encode (Active Files)'. Wait for the green bar to do it's thing and boom, you're done.