A nice piece of advice I've seen is to start in black and white (and shades of grey) and get the contrast and lines and so on right first. Once that looks good, either leave it, or then add colour and texture (and maybe remove some of the lines and borders if the colour or texture changes make them unnecessary.)

Sketch designs on paper on a grid, with a nice pen. It's easier to try out ideas and evolve when they only take a minute.

Practice.