I used some software called "ASA Prepware". I took about six weeks learning the material - actually knowing how to do the weight and balance and stuff like that. That's the old school, and much more difficult, way to do it.
Some of my friends used a website called "Sheppard Air". (not sure about the spelling.) I think you pay a fee to have access to the site for a month or so to study the questions, then you go take the test at a CATS testing site.
Probably the quickest and easiest way is to use an ALLATPS flight training center if you happen to be near one. You can get the studying and the test done in a matter of hours. They sit you in front of a computer where you spend as much time as you want clicking through the questions and learning (or at least memorizing) the answers.
I hear Gleim has good products also but I don't have any experience with them.
Not necessarily the way to go if you want to actually learn the material...
I feel that the test are just a hurdle that you have to jump over to take the practical and are a waste of time. If you have a good CFI and attend a ground school then you will learn the material needed. As far as the ATP test I agree that you should just hit the Gleim hard for a couple of weeks and knock out the test.
But honestly, do you need to know how to do weight shift equations in a DC9 or figure out the max range using charts that you can't even read?
Some of the 121 ops questions are useful things to know, but that is stuff you will learn as you need it.
I took mine out in PHX at Willie and by the time I drove back to Skyharbor and was on the rental car bus back to the airport I think I had dumped all 300 questions I had memorized.
Honestly dude, I would just buy the GLEIM ATP book and study the two weeks and learn the question bank. You'll be covered for every test question. After all that, I would recommend actually reading the sections and learning the info.4