This is one of my favourite videos on how to bench by Jim "Smithy" Smith of Diesel Crew. Its almost 8 minutes long, but it is worth watching.
How To Bench
This is also an excellent article by Eric Cressey on how to bench correctly. Give it a read.
The Seven Habits of Highly Defective Benchers
If you watch Jim's video, and read Eric's article, you will start to appreciate that the Bench Press is a TOTAL body lift that requires good leg drive, upper back strength, and the ability to keep your shoulder blades packed into the bench.