"I like to think of different techniques as 'tools' rather than 'rules'" said bert.
I think of them more as tricks. No rules, lots of tricks.
Or if there are rules, they are like the Ferengi's "Rules of Acquisition", where, for any rule saying one thing, there's probably another saying the opposite. ("War is good for business", "Peace is good for business") Or like proverbs, where "many hands make light labour" and "too many cooks spoil the broth" and "he who hesitates is lost", but "fools rush in where angels fear to tread."
Any rule you can make about how it's done promptly falls down, because there are times when the opposite rule works.
Bob Dylan once said, when asked how long it took to write a song, that it was quicker if you had a good pen, because you could get the words down quicker. And everyone thought he was joking - but for some songs that is just how it is. Songs where you aren't so much making the words up, but catching them out of the air before they get away.
Computers are handy for that, because you can put all the repeats in quicker than you could writing, and you can move all the verses and the lines round and round, and put down the line you know is going to come in somewhere down at the bottom out of the way, until you can find a place for it.
But there are other songs which wait around with just a line or so, and gradually add on others over a long period.
More often than not what happens for me is, I get a phrase stuck in my head, and the song builds around it pretty quickly, and I get it down in writing, or up on the screen. And then it changes, and keeps on changing, in little details. A couple I've kept an eye on, and I've found that there isn't a line or a phrase that hasn't changed. Quite likely the phrase around which the song grew will be long gone.
Rhymes are much less important than rythyms. You can get away with all kind of half rhymes and assonances and things, or no rhymes at all. (One trick that can help if you want to get in a tricky rhyme, with one of the words being a bit unusual, is to use the unusual word first, and the more normal one second. Sounds less forced somehow. Unless you are going for a jokey rhyme, when it'd be the other way round.)
But the syllables have got to fall in the right places (which might not be the same places in different verses - the idea that tunes have to be the same for all verses is a barbarism instituted by lazy folk song collectors).
The trick is to do that without the language feeling out of keeping. I like to feel that the lines I end up with could be used in conversation, which doesn't mean they can't be reasonably high flown at times, because there's room for that in converation.
Rudyard Kipling wrote something which I think applies here, and in lots of other places too:
There are four-and-twenty ways
of constructing tribal lays.
And every single one of them is right!"