I didn't think that there was an easy way. The info that many gave was right on the spot to me. If there is a trick to it, I'd be surprised and glad to know it. This is my idea, though. When I was working we were often asked when a difficult task had to be done: "How do you eat an elephant"? The answer is, one bite at the time.
Pick one key other than the one you know very well. Pick a popular one such as Ab, etc. Learn everything in that key as well as you know it in your original key. It'll take, maybe, three months. Still transpose everything else. Songs in the two known keys, play in key. Then pick another popular key. Learn everything----------. Play in those keys and transpose everything else.
Be sure to learn each key very thoroughly. As you learn new keys, you will find it to be easier to internalize them. The number system is very helpful when doing this. You'll learn four new keys in a year, but you hurry the task by taking your time and learning them well.