Be brutal, get better
In writing, you must kill all your darlings
William Faulkner
So too in game design.
The TL;DR
Listen to feedback and seek out criticism
Remove all the 'pretty', does it still work?
Cut, cut, keep!
Getting Feedback
Get your game in front of people as often as you can, be passionate and excited, even desperate to have people try it out. Once they've played it, you must ask them questions!
If you don't, it's like going to the cinema, paying for a ticket and popcorn, then turning around and going home. Yeah you took all the actions, but you kinda missed the point.
You don't need a fancy questionnaire or an online survey, you just need some way to take notes, a willingness to listen and questions to ask. There's a host of questions you could ask, but for me this is one of the best:
What didn't you like about the game?
It focuses on the things you might want to change or improve. There's not much you can do with positive feedback:
Play tester: "This part is awesome!"
Designer: "Great! So I'll just leave that as it is
then..."
And while it's a great ego boost, if people really enjoyed a game they'll tell you that without you asking.
The question also invites criticism and tells your play testers it's okay to be negative, something most people are uncomfortable with. It also says "the game" not "my game", making any comments about the game, not the designer.
Once they start answering (and I'm talking as much to myself as anyone else here), SHUT UP!
If you've asked a question, let the other person talk - this might just be a rule for life. If you constantly interrupt or correct you run the risk that a playtester decides you're not listening, don't care what they think or are too fragile to hear any criticism. Do this and they'll avoid trying your games in future, or give you pointless, ego appeasing feedback from now on.
I'm so Pretty!
Don't wait until a game is completely finished, for shiny components or proper artwork. Get together the minimum it needs and get people to try it out! Check the rules make sense, that the turns flow logically and the scoring works.
If you've already taken the game beyond that, take it the other way. Remove the minis, take away the cool artwork, strip out the custom dice, peel out as much of the theme as possible.
Is the game still interesting, do you still enjoy it?
Unfortunately there are several games out there (I'm looking at you Fortune and Glory) where a cool theme sits on top of a game that could have done with a bunch more development (and in F&Gs case a rule book clean up).
Cut, Cut, Keep!
Just because you have a good idea doesn't mean you need to use it now, even if you like a mechanic it might not be right for the game you're making today.
Cut away everything that isn't bringing something unique and interesting to the mix.
While you should absolutely trim the excess out, don't throw away ideas, they may not be right for this game, but maybe they'll work in the next one. The same goes for spare components!