A lot of fishermen don't get a chance to spend as much time on the water as they'd like. I was in this proverbial "boat" for a lot of years. There are things fishermen can do when not out fishing that can help them increase their enjoyment and productivity when on the water. Just like a golfer can practice putting or driving while not on the course, or an athlete can work out in the basement, there are a few things fishermen can do.
Nearly all fly-fishermen I encounter can benefit by working on their casting. I have been spending a lot of my freetime practicing my casting in the backyard. I recommend exercises found in George V. Roberts Distance Fly-Casting video. If you can cast well with your right arm, try your left and vice-versa. Just 1/2 hour a day can yield impressive results.
Casting accuracy with spinning gear and baitcasting gear can be worked on. It's amazing how few fishermen can place casts accurately. Not being able to hit a target can cost anglers a lot of fish. I've been working on my underhand casts and pitching and flipping.
Working on tackle always pays off. Going through tackle bags and boxes and replacing old or sharpening dull hooks can be boring, so I like to put on some music or listen to a ballgame and do a few at a time. Respooling reels and taking gear inventory can make the season go much more smoothly. All these things help. Reading fishing magazines and/or books, going over maps and surfing the web for fishing info also add to the knowledge base. Plenty of good stuff on casting, lure-making, fly-tying and more can be found right on YouTube.