The first immutable rule is there must be 140 characters or less and this of course implies that you must be working digitally; type or hand write a draft by all means but it is the ruthless counter at Twitter that will ultimately OK what you put down. This might encourage a syllabic approach where tweet poety has a fixed number of syllables no matter the stresses in the line, Marianne Moore might be used as a model
Second rule is that there can be no returns or paragraphs, and no line breaks other than those shown by the space character or (if you must) the forward slash; this is where the haiku/tweet poetry analogy breaks down for me. A haiku in Japanese would be written in a single vertical line; the English version is usually divided into three phrases (often lines) so perhaps tweet poetry is truer to the original Japanes presentation. But the display area in Twitter being what it is, you will see line breaks where the text wraps; and this acts as an uncontrolled counterpoint to the haiku.
Third rule is that tweet poetry is of the moment; or gnomic. I think this gives us much more useful avenues to explore with their roots in Nordic/Old English riddles. The approach is much less strict in its form but usefully has a strong stress pattern, poetic technique including balance, alliteration, metaphor, and better still, the caesura or break in the line is part of its phrasal expression. So counting the space character as an unstressed syllable is part of this oral tradition of poetry. Academics have suggested that riddle writers had their own grammars, distinct from that governing regular speech which frees contemporary writers to formalise poetry under the twitter restrictions.
Of course, there may be no rules at all in which case I am joining the Dylan Thomas, Chus Pato, Walt Whitman school of tweet poet...
