Haiku is plagued by rules, some of which are false. Others are little-known but essential.
False: the idea that a haiku consists of 3-5-3 lines. I'll let Gabi Greve and Haiku Society discuss that. In Japanese, there is a count, but it's not exactly English syllables.
Fewer people know about the kireji, or "cutting word." In Japanese, these act sort of like punctuation or add grammatical structure, and function similarly to the volta or turn in a classic sonnet. The kireji cuts the poem into two parts. In Japanese, a kireji may indicate a question, emphasis, surprise, completion, probability, cause, or interrelationship.
In English, these words are sometimes represented by punctuation, such as colons, dashes, commas, ellipsis, exclamation points, or question marks (: — , … ! ?) or words like but, how, and, yet, now, this, still (and a lot more) or simply by a line break.
Kireji link two ideas. The best haiku have more depth than a simple observation; they link an observation, often about the changes in nature and its seasons, to something else.
Kireji also free the poet from the constraints of a sentence. Words can be left out, and fragments of sentences can be connected to create both emphasis and brevity. (If you love 17-syllable sentences, you may wish to investigate the poetic form of American sentences.)
How does this work? Here are some of my haiku, offered humbly to avoid plagiarizing other better poets:
hard to say:
which day did the robins
Christmas eve —
the woman in the checkout line
blinking back tears
back to school —
even the playground trees
thinks no one is watching
(Here and is the kireji.)
an empty bench
and a bag lunch
(It's and again.)
dead on the sidewalk
but Spring does not pause
on last year's poinsettia
a red leaf
a girl climbs the playground fence
(Here the kireji is anyway. The location at the end of the haiku brings you back to the beginning, and gives an emotional closure to the poem.)
These haiku also include the traditional immediacy and personal experience — I saw all those things and was inspired by them. Something happened in the context of something else that created a meaning beyond the words themselves.
Recommended resources for learning more about Haiku:
World Haiku Review archives
Happy Haiku Archives
— Sue Burke
Also posted at my professional writer's website: http://www.sue.burke.name