Rhythm: the invisible half of layout
Consistent vertical spacing is felt long before it is noticed.
One unit, repeated
Pick a base unit — 8px is common, but the number matters less than the discipline — and make every vertical gap a multiple of it. Rhythm is repetition the reader never has to think about.
Space belongs to relationships
Gaps encode grouping. A heading closer to its paragraph than to the one above it says, without words, that they belong together. Get this wrong and every other decision has to shout to compensate.
Break for meaning only
A larger gap should mean a new section. If it means "this looked nice here", the reader learns that spacing is meaningless and stops using it to navigate.
Gaps encode grouping. Space is the quietest instruction on the page.
Audit a page: list every vertical gap in pixels. Round each to your base unit and re-look. Most pages have twelve values where four would do.