Only one instance of a form

Does anyone have any suggestions as to how to assure that only one instance of a form is opened?

Yes, but you have to keep up with the form yourself. You can declare a Public Shared variable on the form and set it to true when you open a form, and set it to false when you close it. Then, check this shared variable to see if the form is opened.
Other than that, you'd have to pass references for all of your forms.