When you first "get it," you feel excited about the new technique. Perhaps you found out what you've been doing wrong with your backhand, or someone pointed out that you need to wait longer before hitting the ball. You managed to keep these things in mind for a while, but soon you were back to your old ways.
Sometimes, we really do turn over a new leaf and it grows into a tree (poor metaphor, but hey). More often we return to old patterns without ever meaning to. So when they do stick, and what makes the difference?
The Missing Element
We don’t have a hope of keeping a new technique unless we use it in regular practice. The old conditioning of your muscles and your mind is too strong for your mindful desire to change to be enough. You must condition that new technique until your muscles and mind don't remember the old way.
You Need to Practice
Just a short amount of mindful practice of your new technique every day is enough to get a foothold and start to dissolve the reflexive nature of the old behavior. It isn't a good idea to focus on more than one thing at a time. Give your mind and body a chance to engrain the new pattern before working on something else.
It might be as easy as taking five minutes out of your morning couples game to have your partner reinforce the new pattern, either by hitting balls to you or watching to make sure you're returning them correctly. No doubt your friends could use practice, too, so you'll all benefit.
Doing even a little bit of the new technique routinely starts to break down the automatic nature of doing the old one. The new way happens without thinking about it more often. Gradually, the new way becomes natural.
The "AHA!" moment is only the first step on the road to improving your game. You really change when this new way of playing feels natural, and for that to happen, you need to practice.