Besides the fact that it's popular, Le Cellier has relatively small seating capacity.
No tip for getting an ADR is sure fire, but here's what I would do:
Hopefully you're staying on site so you can take advantage of the 90+10. Pick a day for Le Cellier that is in the last few that you'll be staying at Disney. This way, fewer people have had the chance to make ADR's for those days.
I had a lot of problems with the online system Sunday when I tried to book. So I called at 7am when the phones opened and I was able to book Le Cellier - for the last night of my 6 night trip.
Otherwise just keep checking the online system any time you're near a computer. Once in a blue mooon someone cancels. I was trying to get in to Le Cellier for a trip in October that was planned less than 90 days out. I just keep checking for Le Cellier. One day, I actually found an opening for lunch, but the day and time didn't work for us so I didn't book it.
Good luck!