For general character design, focus on silhouettes first, then later think of each detail on an outfit or how big you want the body parts to be, or how weird the haircut must be. Always do research even if your character has no basis in reality - animal anatomy, clothes, weapons, everything to get the ideas rolling and have a good reference to look at while you sketch. As mentioned before, never stick with the first sketch, even if you have to give it some time to think of what needs to be changed.
For comics, focus on character designs you can reliably redraw. If you have the whole week to yourself and no schedule, sure, go ahead and make an entire tapestry on your knight's cape, but most people won't have it and it's better to have some stripes or on striking color. Your time and hands will be better spent if you have sketches or a character sheet to know how they'd be seen from various angles too, rather than having to figure it out on the spot.