I've found that Rob's manifest entries did not do it for us... We still ended up without "Do not show this again..." check box showing under 1.7.0_51 for our app (when href= was not specified, properly signed, etc.).
But other of what Rob posted holds & that coupled with Andrew's suggestion of a full/proper manifest lead us to what worked for us. From my post to Java Web Start security dialogs with Java 7 update 51:
Assure you've JAR manifest(s) such that no Missing Blah-Blah-Blah manifest attribute msgs show in console log. The minimal additional manifest attributes for 7u51 we've found must be present (*s as test values):
Permissions: all-permissions
Codebase: *
Application-Library-Allowable-Codebase: *