There are many good casting videos out there, and some pretty bad ones too. Although I am no longer a fan of Lefty over his association with Donny Beaver, I can not deny that his videos have been a huge help over the years. The late Mel Kreiger has a good video in which he shows common mistakes, tells you what caused them, and then how to fix the problem. If a local shop or FF'ing group offers lessons I would recommend taking some. As others have said, once you have a bad technique ingrained it is very hard to un-learn it.
As a guide I see hundreds of fishermen every year. The most common casting error is trying to start the cast with slack in the line wth the rod tip too high. After this it is a toss up between not generating enough line speed and not pausing the correct amount of time between casting strokes.
When practicing we all have a tendancy to make as many false casts as we can, until the whole thing falls apart. Often this comes from focusing on distance and not good casts at 20 feet, then good casts at 25 and 30 feet. Do not worry about distance casting for the vast majority of trout fishing. Even on our big rivers in Montana the guy that can cast well out to 30 feet and get a good presentation will almost always out fish the guy who flails away, trying to cast as far as they can. This is even more true in Pa.
Maurice nailed it when he mentioned exessive false casting. Next to overall bad casting techniques, IMHO, exessive false casting causes more problems, tangles, spooked fish, flies in the bushes, and lost opportunities than anything. As you perfect your casting also focus on reducing your false casts.
One last thing. Someone mentioned that the western guys, those who learned out here, are often no good on smaller, tight streams. That is so right! Spend time on small streams where you have to watch your backcast. Learn to focus on a very small target for your flies. Smaller streams really make you focus on your casting, and that really helps on bigger water.