This is a simple heuristic that I have found useful when designing things.
If a design is subpar in a low resolution format, adding more detail will not improve it.
Let me give you some examples of what I mean with that: A design that looks bad as a doodle, will not improve by being turned into a detailed sketch. A design that looks bad as a sketch, will not make for a good 3D model. A 3D model that looks boring, will not be improved by adding fillets. And so on…
While this might sound obvious I think it is good to be conscious about this heuristic. Otherwise it is easy to fool oneself into believing that the design concept you have will become great later on, once all the bells and whistles are in place.
If we take a look at some of the greatest products ever created, we see that they looked good in very different stages of the process.
An important caveat to all of this is that the designer is the judge of weather or not the design is in a state that merits moving forward with. There might be cases where a sketch looks bad to the outside world, but the designer sees hidden potential.
To summarise: Keep doing what you are doing until it the design sits right with you, and only then move onto the next step.