ALWAYS be making games! Make bad ones, make worse ones, make better ones, make good ones, make great ones. Just keep making games. Sometimes making a game only takes an hour. That doesn’t mean you will finish the game, but you made enough of a game to know how far you want to take it. You can take it to prototype stage, play test phase, production, or you can junk it after 10 minutes. The important part is that designers are constant using different mechanics and themes in different way. Game design is an art form and we need to practice that art to make great games.
But Chris, what if I only want to focus on one design? Practice using different mechanics to do the same thing within the theme of your game. Certain mechanics can accomplish different goals you want your players to experience. When you practice using different mechanics you will teach yourself what mechanics work best for which parts of your game. If you are the kind of designer with lots of ideas, you will be able to use your practice to utilize various mechanics and themes in other games down the design road.
Mechanics and themes have sweet spots and we designers need to find those sweets spots so we can best utilize all the tools we have at our disposal. We need to learn what tools work in different situations and we can learn these things by the practice of making games.