Study McGonagall! The basic principle of this kind of brilliant poetry is to spew out a line about anything at all that crosses your mind in regards to the subject you want to focus on...and then search about for any possible rhyming word with which to conclude the next line. Just keep doing that for about 10 or more verses, until you think you've said enough.

You will find that certain words, like "declare", are very handy when using this technique.