All of this stuff sadly depends upon things outside of your control. If you really want to optimize video, check out http://www.avsforum.com
--those guys are nuts for video the way the a'gon users are nuts for audio.
The problem with CATV is that a lot depends on the signal to noise (S/N) ratio of the signal delivered to your particular house, which depends upon the quality of the CATV plant in your area. Its not even CATV-provider specific--some people get great S/N with Comcast, others hate it because they are the tail of a looong run with lots of repeaters, others love Time Warner, others hate it, etc. The AVS guys can tell you how to access functions on your cable box to figure out what your "delivered" S/N is--if its really low, tightening all the connections may in fact improve it. If that doesn't work, a call to the CATV company may net you a tech--if the quality is bad enough, it may motivate them to upgrade some upstream plant to get you a better signal.
The problem with satellite signals is that it depends upon the view you have to the satellite and whether you have a good antenna install. Again, there are ways of checking on the box the quality of your signal. That won't help, however, if you have a big 'ole oak tree with foliage 3/4s of the year that obscures your southern view--these sats are in equatorial orbit, so the view to geosync orbit is more "up" the further south you are. If you are in the extreme north, basically you are screwed b/c the likelihood there will be a tree or building in the way is increased substantially. Weather also affects the signal. I used to laugh at a device I once saw--a little umbrella to protect your dish from the rain. Draw a line from your satellite dish to 22,000 ft above the equator. Any rain in that path--which could be miles away from you (hell, you could have clear skies overhead) may render your signal unuseable.
The problem with over-the-air (OTA) is that it depends upon the quality of your antenna, terrestrial blockage, and where the transmitters are. You can go to http://www.antennaweb.org
(or at least you used to be able too--I haven't checked in a while) and see the gain of antenna you need to get particular HD signals in your area. Some AVS freaks have arrays of directional antennae they use to pick up different signals. You will also need an OTA tuner device to take the digital signal and turn it into something you can get into your monitor via RGB or DVI. (Newer models should have OTA tuners built it if they are sold as TVs).
So, there are ways of tweaking yourself out of some of this stuff. In reality, the only thing you can blame on the CATV provider, broadcaster, or satellite provider is sucky content. But, that is a different thread. ;)
Hope this helps.