As everyone else had mentioned, set the CausesValidation property to false.
<asp:Button ID="MyButton" Text="Go Back" CausesValidation="False" />
This is straight from the Microsoft Help Page:
By default, page validation is performed when a Button control is clicked. Page validation determines whether the input controls associated with a validation control on the page all pass the validation rules specified by the validation control.
You can specify or determine whether validation is performed on both the client and the server when a Button control is clicked by using the CausesValidation property. To prevent validation from being performed, set the CausesValidation property to false.
Note:
You should set the CausesValidation property to false when you are using the PostBackUrl property to post back to a different page. You should explicitly check validation when posting back to a different page. For an example, see the Remarks section of the PostBackUrl property.
This property is commonly set to false for a reset or clear button to prevent validation from being performed when the button is clicked.
When the value of the CausesValidation property is set to true, you can also use the ValidationGroup property to specify the name of the validation group for which the Button control causes validation.