Reading a lot of good copy certainly helps. I remember reading at Gary Halbert's site his suggestion that you physically re-write a sales letter in full as many times as it takes to "get it." Staying up late working on the computer with infomercials running in the background is also a help.
Nothing will make you a better writer than . . . writing. Doing the work. Putting in the hours. Dropping the adjectives. Editing your work like it was scrawled in crayon by your worst enemy. When Leon Uris was asked to speak at a college to a writing class and impart some of his pearls of wisdom, he opened the talk with: "Who among you want to be writers?" Naturally the whole class shot their hands into the air enthusiastically. "Then why the hell aren't you home writing?" Uris growled. He left the stage and went home. That was all he had to say on the subject. And that pretty much sums it up. No one can tell you how to write. You will find your own voice and your own style by doing the work. So why aren't you writing?