"A sideline warning has been issued to East Westburg State. No yardage will be penalized with this warning."
Certainly not bad or wrong, but it suggests that discretion was used in not penalizing.
My suggestion (and what I use) is:
"Sideline warning - State University. This is a warning, with no penalty. Additional violations will result in a distance penalty."
This makes it clear there is no penalty associated with this warning. The last part doesn't really threaten, it just states the fact that, should whatever caused the warning happen again, there will be a penalty. We can't help if some folks see a threat in this. They are the ones that only respond to threats, anyway.
This way, no one is surprised when the next violation, indeed, results in a penalty. It will mitigate the, "You didn't warn us" or "We get another warning," arguments.
Remember, if you ever get to the 15 yard penalty, either with contact/interference (even on the opening kickoff) or by violation attrition, the warning and five-yard penalties are no longer available. Every violation after the first 15-yarder is 15 yards.