I recently bought a Sony 46" W series LCD after extensive research online. Pity that this didn't come up a few days ago, as I just threw out the research links I had.
The first to do is to find out what features that are important to you. I started with Google searches for flat screen TV comparisons, flat screen TV reviews and flat screen TV ratings. Be prepared to plow through a lot of vendor links. When it came down to final selection, I found sites that had user reviews and ratings on specific models the most useful since real world user experiences can be more telling than expert reviews and the reviews you find on CNet and other sites tend to be limited to a few models and are often dated. Not that I didn't read all of the expert reviews I could find. Reviews on one size TV of a model series will usually apply to other sizes, so don't worry if all you can find is a review on a 46" set when you are looking for a 42"
As others have said, LCD TVs are a better than plasma in a bright room. They weigh less and use less power for a given size than plasma. I was able to mount the 46" Sony LCD on the wall by myself, while there is no way I would have been able to do that with the same size plasma. I was looking for a TV that supported 1080p, and there are a lot more LCDs than plasma TVs supporting this. Plasma does have deeper blacks and handle high speed motion better than LCDs and have a wider viewing angle than LCD TVs, but LCDs have gotten much closer lately.
One thing that became apparent in my research is the importance of the image processing system of a high definition TV when it comes to viewing standard definition sources. Inexpensive sets will often look very good displaying high definition sources, while standard definition can look horrible because they don't do a good job of "upconverting" lower definition sources. Higher quality sets do much better. Higher end LCD sets generally do better with action scenes.
Don't waste money on high end cables. Tests have shown no benefit. If you are planning on wall mounting the TV yourself, Sam's Club had the by far the best price I could find on a TV mount. They also have good prices on cables.
Of course, as with any purchase of this type, the first thing you need to do is set a budget.
Once I've settled on a model, I hit the shopping sites like Pricegrabber.com and Bizrate.com and look for the best delivered price from a highly rated store. I rarely buy accessories from the same company as the main unit, since it seems that the stores that have the best prices on the big ticket items try to make it up on little stuff.