What is the simplist way to always ensure that data is entered into a textbox on a form?

If the textbox looses focus, I want to alert the user to the of the requirement. Likewise, I want the user to be notified of this requirement when they try to save the record even if the textbox has been skipped and never received focus. In other words, I want to prevent the user from saving the record until data is added to the textbox.

I've made a couple of attempts writing code to check if the .Value of the txtbox is EMPTY() but this isn't working too well. Is there a better way?

Thanks,
Robert