Another approach is validation, which in my opinion is better. You can provide all the tips you want and still get garbage in. Validation would not only prevent that, the validation message would provide the necessary message/example to get the right data. That way, once they get the hang of it, they won't get corrected. A mouse over event will fire even if you're just moving the cursor over to a button or some other field and happen to cross over one that has the mouseover event on it. THAT will get you some new nicknames around the office, I'd say.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.