So formalism uses regular meter (for example, iambic pentameter in Shakespeare) and often rhyme. What other rules are there for writing modern formalist poetry?
I’m still learning the answer to that question. As usual, it’s easiest to start with the list of what you shouldn’t do. There are some definite turn-offs that I’ve learned from Stephen Fry’s book, from workshopping poems online, and from helpful poetry journal editors such as George Simmers at Snakeskin, who has posted some guidelines (http://homepages.primex.co.uk/~simmers/). Here’s a partial list.
1. No archaic words. Just because poetry forms like the sonnet go back a long time doesn’t mean we should be writing them like we’re wearing periwigs. Old-fashioned terms like o’er and ‘tis and droppeth belong on the dustheap. Modern poetry should use modern language.
2. No inversions or other tortured syntax. Nothing screams ‘amateur’ like sentences that are twisted around to fit a rhyme: “I know that I will disappointed be / each time a garbled sentence I see.” This isn’t how we talk in English. Poems don’t need to be exactly like spoken conversation, but the grammar should be roughly similar. If your poem sounds like Yoda is saying it, then an inversion you have. Fix it.
3. Meter should be regular but not too regular. This is a hard one, and I’m not sure I get it yet, but if your poem trots along da-dum-da-dum-da-dum-da-dum-da-dum with absolutely perfect regularity for 10-20 lines, it can be monotonous, especially if there is a full-stop or a comma at the end of each line and never in the middle. Vary the rhythms a bit with caesura (a pause in the middle of a line instead of the end), and with the occasional metric variation. These can include extra unstressed syllables (da-dum-da-da-dum), or missing stress (da-da-da-dum), or reversed feet (dum-da-da-dum). But, well, this seems to be a matter of taste. Some people always seem to read metrical variation as a mistake, while other people insist on it.
In general, I think the golden rule is to make it look easy. The language in a poem should seem as free and natural as conversation, even if you’ve had to sweat bullets beating it into shape.