Try to get your ear as close to the leader of the song or tune as possible, so you can be sure you are exactly with his or her rhythm--particularly essential if (as sometimes unfortunately happens ) not all the instruments in the group are exactly together in the rhythm.

3rds and 6ths often work if the melody is in a major key, 4ths and 5ths if it's modal.

Number 8 of your list is by far the most important--don't play at all until you're absolutely sure you know how the tune goes--and if you by some chance still guess wrong, be willing to change immediately. There are some accompanying instruments for which a mistake can sound like a little flourish or a leading tone----if you change to the right note at the speed of light.