For the tall grass eggs, the message you get when you choose the
wrong spot can help you out quite a bit in figuring out where to
search next. The Tall Grass wiki page has more specific
information on that under the "Searching the eggs" heading. If
you're searching for a higher rarity pokemon with a big field so
forgetting where you've searched with what results is more likely,
taking a screenshot of the field and marking down your guesses and
the messages you get on it with paint (for example) can be a useful
way to work on a strategy to hunt down that egg.
When it comes to rumbling, as far as I'm aware, there's no way to
boost your chances. You're at the mercy of luck there.