A form is a conversation, and most forms talk too much. Every field you add is another reason for someone to abandon the flow, so the first design pass should be about removal, not layout.
Inline validation beats a wall of errors on submit. Telling someone their email looks wrong the moment they leave the field respects their time and keeps the correction cheap.
Finally, never make users guess what comes next. A clear primary action and an honest progress hint do more for completion rates than any amount of visual polish.