What I do is on the days I'm not sure where we will be I try to book at non-park locations. That way we don't have to use an admission to just eat (we won't). This usually works well for us. When I make our ADRs I try to guess where we will be. Of course, things don't always work as planned and I know this going in. When I'm at WDW as soon as I know we won't be using an ADR I call to cancel it as well as to make a new one. We've always been able to get an ADR somewhere. Raglan Road, Narcoossee's, and Kona are some we have gotten last minute (day of,etc.).
So, like many have posted, don't make multiple ADRs for about the same time. You may end up losing them anyway and it's really not fair to others. There will be certain ADRs that you know you will keep, no matter what (those requiring a deposit, etc.) and there will be those that you can be more flexible on. Realize that many guests are the same as you, their plans change. People do cancel ADRs. Even the day of you can usually get an ADR somewhere nice. Of course, the earlier in the day you try, the better your chances. I remember a few year back being on the phone in the a.m. with Dining and making a change. I wanted PF and nothing was open, then she said "Oh, something just became available-looks like there was a cancellation." This just emphasizes why cancelling ADRs is so important as well as trying to get new ADRs does work.
Good luck and happy Dining!