For the first couple of years I was doing longsword, I had real trouble keeping my arms extended and pushing my hands up (due to a lack of strength, lack of endurance, and lack of the habit).
I did all manner of training to work on this—exercises for arm strength, especially overhead pushing, endurance training for those same exercises, and of course sparring to train the habit. (See in particular Fitness training for longsword.)
I’m not there yet, but it no longer seems to be my worst problem. Here’s a sparring match with one of the better fencers in our local group:
I’m not quite all the way there, so it’s a thing to keep paying attention to, but it’s no longer my biggest problem.