What worked for me: I learned to draw. It took a lot of practice as I really sucked initially. I recommend getting a notebook with grid lines and a ruler.
It takes a lot of patience to pay attention to all of the details but it pays dividends. With drawing as an intermediate step between having something in my brain and having something in pixels I'm much better at capturing a vague visual concept, iterating, and bringing it to life.
I suspect this process will be different for everyone- this is just what helped me (after years of limping through the design process).