Retros are my #1 as well. Retros can derive the rest of the changes that need to be made, and if you're doing them right, the team will be making the changes together.

My next two are:

#2 Never say "Agile" again. If you can't explain an idea from first principles without defending it as Agile then you're not ready to promote it.

#3 Never suggest a change that isn't a real solution to an actual major problem. You're only going to lose supporters if you're continually trying to get them to do things that don't help them.

If you do these things, you'll have a much greater chance of success, though the end result might not look anything like Scrum, XP, etc. (and that's okay).