In early 1999, I decided to upgrade from Novice to Extra. I had already been a ham for a long time, so the only study material I used was the practice tests on
QRZ. com. I just read through all the questions and started taking some practice tests. The practice tests are nice because they keep track of your progress and mark anything you get wrong, for further study. When you are getting a 85% - 90% on each practice test, you are ready for the real test. In just 3 months, I went from Novice to Extra. If I waited a couple of years (2007), I could have skipped the 13 and 20 WPM Morse tests, but I opted to take them too.
As someone else mentioned, the real learning happens once you get the license. Many wannabe hams are able to take all three written tests (Tech, General, and Extra) in a single sitting. It's worth a try, if your up to it.