Oof! I am, of course, in that list of shame.
But anyway, quite a lot of options:
Title matches are prioritized – when in doubt, search for titles
Unique, uncommon words will produce the best results
Try searching within a particular category, topic, or user
order:views order:latest order:likes @username user:foo
status:open status:closed status:archived status:noreplies status:single_user
#category-slug category:foo group:foo badge:foo
in:likes in:posted in:watching in:tracking in:private
in:bookmarks in:first in:pinned in:unpinned in:wiki
posts_count:num before:days or date after:days or date tags:one,two
rainbows #parks will search for topics containing the word “rainbows” in the category “parks”.
rainbows category:parks status:open order:latest will search for topics containing the word “rainbows” in the category “parks” that are not closed or archived, ordered by date of last post.
rainbows category:“parks and gardens” in:bookmarks will search for topics containing the word “rainbows” in the category “parks and gardens” that are bookmarked by you.