Dashboard filtering question

I have a dashboard that has 3 prompted filters where the default values are nulls or spaces and the report runs automatically and returns a large number of records.

I have two questions

1) How do I keep the report from initially running until the user enters a value into one or more of the filters or just hits the "GO" button

2) Is there anyway to create a presentation field on the dashboard that is not dependant on a table field that I can add to the report as a filter?  example:

have a presentation variable with a default value of "No" that I can then set to "Yes" and the underlying report only runs when the presentation variable value = "Yes"