At Haiku North America, I participated in a short renku class moderated by Kris Moon, co-founder & past president of the Association for International Renku (AIR). As a class, we wrote four short renku of four links each, and began to explore the techniques involved in linking and shifting in a renku.
On the surface, renku seem simple. Start with a haiku, follow it with two lines, then another haiku, then two lines, and back and forth until a long stream of linked pieces occur. However, it is not so simple, as there are many cultural rules that must be followed.
For example, the renku progresses seasonally, and follows a strict pattern of seasonal flow, as shown in the charts here. Furthermore, the fifth, thirteenth, and twenty-ninth stanzas MUST mention the moon, and the seventeenth and thirty-fifth stanzas must mention blossoms of some sort. On top of that, various stanzas are required to address the topic of love. Even more complicated, stanzas are not permitted to repeat key words or phrases from previous stanzas, so every new link must be fresh and poignant.
Because renku progress through the seasons, a good saijiki--collection of kigo, or season words--is recommended. William J. Higginson's Haiku World and Haiku Seasons are good resources to start with. Gabi Greve's World Kigo Database is extensive and online, but difficult for some to navigate. Kris Moon maintains a list of 500 Season Words that can be a good starting place for renku writers, but many of them are Japanese oriented, and not appropriate for Western readers. However, all of these saijiki, as well as the many others available, provide a beginning writer with a starting place on how seasons are organized and treated in traditional Japanese poetry.
Geoff Landis has recently started a Cleveland Poetics Renku, which can be interesting due to all the chiboo kigo associated with Cleveland, so I strongly urge folks to participate. However, though experimentation and play are essential to the growth of poetry, I would encourage poets who participate to familiarize themselves with the form and its rules, and to read the copious examples available online. William J. Higginson's Renku Home is a good place to start, as is the archive for the Bernard Lionel Einbond Renku Award. I can't wait to see what sort of renku Cleveland comes up with.