Work on your fitness, as the better your fitness, the longer you can bat and concentrate. Obviously, doing some reps and working on your biceps and forearms will help you get stronger, but if you're not physically at your best, then you will tire easier when using a heavier bat, and your timing will go off. I jumped from a 2.10 to a 2.13 and a half when I was at a decent fitness level, and was fine with it. But as I slacked on my fitness, I noticed that I was not quite as comfortable with that weight as at first, so now I'm only using up to 2.12 and a half. The way I see it is that most people could use a heavier bat around 2.12, but not everyone can sustain it over a long period of time. Sometimes, you gotta just bear through with it, and if you are 100% sure you want to start playing with a heavier bat, then don't swap to a lighter one at all during net sessions. It's the only way to get used to using it over a longer period of time!