Practice, they say, makes perfect but, are you practicing what you want to make perfect? Endlessly doing something wrong is not going to somehow make it right. You have to be mindful of how you row so that what you do over and over is what you want to continue to do when you tire. You want to ingrain that good muscle memory.
This is especially true as we move off the water and onto the erg, as many people think of and use the erg mainly as a conditioning tool. It works great for that but you can also use it to help break bad habits and create good ones. I would argue it's even easier to work on technique on the erg than on the water - fewer factors in play, fewer distractions.
You can easily set up mirrors in front of and beside the erg to see what you're doing, easily get someone to video you or if you're really motivated, set up a camera next to the erg that you hook up to a screen in front of the erg and watch yourself in real time. Any of these will allow you to see what you're doing, unlike in the boat when you often think you're doing A only to be told you're doing B.
Your first step is to identify an area that you want to improve. Is there something the coach is always nagging you about? A part of your stroke that you feel is particularly weak?
The second step is to figure out how to fix/change it. Are there particular drills you find helpful or that your coach has recommended? Do you have a good idea of what you should be doing and what it looks and/or feels like?
The third step, be mindful of doing it right. Start out slow, incorporate some drills, then gradually increase the power. If you get to a point where you can no longer do it well, back off until you get it right.
Remember practice makes perfect but be sure that what you're practicing is what you want to make perfect.