The trick to effective "I" messages is to:

* Address the problem behaviour ("all this noise and jumping around in the back")
* How it makes you feel ("I get nervous and I hate to drive")
* Why it affects you ("I feel I have a right to enjoy this vacation, too")

Then they learn that their specific behaviour at that moment makes you feel xxx which stops you from being able to xxx.

Another example might be:

"David, I feel frustrated that you leave your shoes in the middle of the loungeroom every day after school. I am worried someone might trip over them and hurt themselves (or other reason like it creates more work for you etc)."

This can be much more effective than, "DAVID! I told you to stop leaving your shoes in the loungeroom after school, did you not hear me?" which only serves to build resent.