1. There are no stupid ideas, only ones that must undergo some further tinkering and flexing.
Pro-tip: if working with a pen and paper, place random ideas in the margin of a page.
2. Do not, I repeat, DO NOT be a control freak. I understand that everyone wants their song to be perfect, but you can learn a lot from the other person if you keep an open mind.
Pro-tip: if possible, write down whichever ideas were passed around then leave and come back the next time. By 'sleeping on it' or letting the ideas 'marinate' for a short while, we can grow more fond of those ideas that we contested against beforehand.
3. Try not to overthink but also don't be overly hasty in writing a song. Let the words and melodies flow naturally.
Pro-tip: if you are slow writer like myself or Leonard Cohen, try snatching any sort of idea you have lying around somewhere and take it with you to a collaboration session. Whether your song lyrics are written on tissue paper or a shopping receipt, it doesn't matter, just bring them with you to jump-start your brainstorming.
ALSO...
If you're superstitious and/or in need of immediate inspiration, keep a good luck charm such as a piece of jewelry, a rock, a coin, or any kind of sentimental item. These will help you stay rooted and balanced as you proceed in writing a song that will speak volumes for people.