Self Doubt is my first Tester
I have several games in the works right now, which on one hand diffuses my focus and may not be entirely healthy. But, on the other, all of them are in weird places: long term development, early testing, late testing, rules tweaking, pitching, and so forth. All of that stuff consumes one portion of my brain and the other side, the creative portion, has little ideas with which I like to experiment. Some emerge, some don't.
There is a really good argument to get a game to its playable state as quickly as possible. Come up with an idea, yank out some cards to build it, and test. I don't think that's a bad approach, and for some peers that's the only way they CAN do it, as their mind requires the tangible pieces to move about and consider.
I, however, especially this past year, have found myself moving through a long marination phase of consideration and introspection before I build the prototype or even build the rules. And lately, I find myself building a prototype, then taking another week or so to think on it further.
Self doubt is my first tester and I want to make an argument for such marination to occur. Not to say it's the ONLY way to do something, or even that it's the right way, but merely to suggest that you may find gems by doing such a thing.
Firstly, your testers are precious, or more accurately, their patience. If you're lucky enough to be surrounded by a group of designers, it's possible to bring garbage to the fore and they'll be fine with that. However, few have such a benefit. You should take the time to consider your prototype privately before exposing it to your test group, if only to make a stronger first impression. If your prototype is so mechanically broken that little promise can be seen, you've done yourself and your game a disservice.
Secondly, without a little marination, you, like myself, may find yourself relying on "old favorites" or "old habits." I have a crutch upon which I constantly lean, which is action cards. I love them, but it's stifling my creativity and unique approaches to problems. My first response "this needs spice" is to design 30 action cards. Nein! If you're rushing to prototype, you're only allowing your creative brain to conceive so much. You will fall back on comfortable trends, which again, is a disservice to your design.
My suggestion, is to take your core theme and core mechanic, then quickly build around it. If this includes your crutches, fine. Do it with abandon. Then, instead of taking the game to Billy's, circle back and circle your less inspired concepts with red ink. Quickly list out alternative solutions to solve the same problem. And if something is in your design, you should know why, even at this stage. Challenge yourself from the viewpoint of using a simpler mechanic, or fewer components, or using a specific component to jumpstart the creative process. If you rely on cards too often, use hex tokens instead. If your favorite game in the genre uses a rondel for action selection, experiment with dice instead.
However you go about it, give yourself the time and put forth the effort to do things differently. Let your inner demon speak up and say, "Hey <your name>, can I be honest? You can do better."
My final element to this simple treatise is that you need to be your game's greatest champion. I was listening to Alex Bloomberg's Start Up podcast this morning -- hugely recommended, go grab it. In his first episode he's pitching to a billionaire investor, who notes the most important quality to a company pitching him is their emphatic and devout belief that what they're doing is important and will be successful. They aren't saying "this can be cool" or "we think this is an idea." They say, with conviction, this idea will change the world. This business will make money.
You need to allow self doubt to seep in to challenge your conviction. You need to battle it and emerge victorious. You're going to receive input, especially early on, that your game isn't fun, that it's unoriginal, that it's fiddly, overly complex, or isn't as fun as a recent game from <insert hugely successful designer>. They'll be completely right now, but in the face of that, you need to know why they'll eventually be wrong.
Allow the early criticism into your design from day one. Take the time to address the concerns, enrich the core, and become your greatest fan.