I would recommend the CodeQuick software. You can buy it at: http://www.cq2k.com/
I used this software and within 2 months of using it, I passed my 20 WPM test. Some people will tell you the method it uses won't allow you to go very fast, but I am approaching speeds of 35 WPM in just 4 months on the air. I still "hear" the sound-alikes used in CodeQuick, but don't really think about them. The code just translates to letters and words automatically in my brain. I believe CodeQuick is about 35 dollars. I think you should give it a try.