I have four of them.
One effective way is to have a story, almost a poem, that needs to be transformed into music. Especially with my own lyrics. But it also works with the lyrics of my brother. (www.funktionshuelsen.com)
The second one is to just jam on the piano or the guitar. Most of the time I come out with new ideas. But I struggle to put meaningful words onto these notes. This way of progress takes longer.
The third way is to record original kids words and cut out snippets they said or sang. This, sometimes can be a great basis for songs, in both, either music or writing.
The most effective way I found out lately, is to grab a guitar and go out to the place where I want to catch a certain mood. Either within the place I live or even better, in passionate surroundings like vacations. Mood and instrument together, let's say in harmony- most of the ideas are songs completed.
These seem to be the most common ones for me.