Are you talking about the Fantasmic show at MGM or Illuminations at Epcot? For the Fantasmic Dinner package at MGM you can do as the previous poster said, get vouchers at dinner for a special reserved seating section. Disney will only have early PS times until about 1 month before your date, then they will probably open up some later times. I would do one of two things. 1) Take the early PS and have a late lunch/early dinner that day. We have done a big breakfast buffet, then the early dinner time and skipped lunch on the day we did the Fantasmic package. or 2) make the early PS then call every couple of weeks to see if you can change it to a later time.
If you are talking about seeing Illuminations at Epcot, there are only a few restaurants where you can view Illuminations while eating, but none of the viewing seats are guaranteed.