Walk. Don't just take her outside and stand around waiting for 'it' to happen. Walk her.

Movement is needed to get things going and as we age - humans and canine alike - it takes some time to get the kinks out, become fully awake, and get our insides aware that we are now UP and time to move.

A walk in before bed will also help prevent overnight episodes. A 15 minute walk should do it. Let her sniff when you first go out, and then keep moving for the full time - unless she has to go, of course.