Hi,
In all seriousness, the single thing that works best for DD4 is to make sure she is not tired. Tired=cranky=meltdown
Also, I try not to let her eat anything 1 1/2 hours before we go so she will have an appetite.
Make sure your ADR's are for kid friendly places. (read: noisy not quiet)
Also, you could let them pick out a small, quiet souvenir that they can not play with until restaurant time.
If possible, I always ask for a table in the corner or side of the restaurant. DD4 is usually quiet but is bouncy and the right seat can keep her from bothering other guests.
Practice "restaurant behavior" at home.
Above all, my husband and I are always willing to forgo the sit down restaurant if things don't seem to be at "optimum behavior level."