For designers, play every type of game you can find, including board games, card games, party games. Learn how people interact, how they communicate, how they think. Remember that newbies are easily overwhelmed by lots of choices. Introduce your ideas slowly in gameplay. Focus is the main tool of the designer, but focus takes time, energy, and research.
Most ideas start out far too complex and disjointed.
Ask any game developer what the initial and final play specifications were for any game they've ever worked on and you'll find out what I mean. Pair things down beyond reason. Pair things down so far that you don't even think you have a game left. Maybe then you'll actually have a game. Wolfenstein 3D is an excellent example of this principle.