HiCustomer there's no trick to it, you just have to get lucky. There is a plastic piece that rotates out of position and causes the problem. Try locking the cap again and then pressing in firmly as you rotate the cap very slowly. Sometimes you have to get one side free and then the other. Try pulling out on one side while you rotate. Keep playing with it and you'll get it off. You will need to replace the cap when you do. You can rotate the white plastic piece back around to line up with the metal ears but it will rotate out of place again. Let me know how you make out.
HiCustomer unfortunately there are some problems that just aren't technical in nature and this was one of them. You probably realize that now that you've removed the cap. Rather than ignore your request for help I felt it was better to reply to you with the assurance that you were doing all that you could to remove the cap even though I knew I couldn't offer you any more than that. Using excessive force can damage the filler neck and could result in the cap not sealing thus putting on the check engine light and requiring the tank to be replaced. Your second reply turned the question of the cap into a desperate plea to make an appointment. I tried to offer you a clearheaded solution to your problem. Ultimately you are never under any obligation to click Accept for the advice that was given but please know that most of the experts on this site truly want to help you solve your problem. I'm glad you finally go it removed :-)