Couple of differet methods, depending on how late you're planning to stay at the park. Can you stay late and see the 10:30 show? That will make it easier to get a good spot.
For 9 p.m. show, some people get there at 6, put out a blanket directly across from TS Island, and camp out. One person holds it down, somebody else goes to get some counter service dinner.. they eat, people watch, read, play games... maybe Dad takes the kids to do a couple rides, then switches off with Mom... 9 p.m. rolls around and the place is packed like sardines, but you've got front row seats on the ground (hard though it may be).
Some people shuffle up 10-30 minutes before the show starts and stand to watch. But it's hard for kids to see in the standing crowd, and it is VERY packed.
Some people swear by finding the center light tower, which raises up just minutes before the show starts. It's a big tower of lights that looks like it will block the view, but those in the know hang out right behind it, because it goes up and gives you a pretty good center view without a whole lot of waiting.
Some people pay for reserved Riverside Seating (available for both the 9 and 10:30 shows). It's $59 for adults, $49 for kids 3-9 (lap sitters 3 and under are free). You order tickets 30 days in advance. You pick up your tickets at Guest Services outside the gates the day of (or a couple days before). You check in near the churro cart at 7 or 8 p.m. (it's supposed to be 8 now, but people are reporting that they're still doing it at 7 this week), and first come first served pick out their seats from a seating chart (people start lining up 30 minutes before to pick out their seats, but in my experience they're all good). You show up at 8:30, a CM takes you to your chair, and you get a nice box with mini desserts, truffles, a choc. covered strawberry, grapes, cheese and crackers. The CM brings you your choice of coffee, hot chocolate, water or soft drinks. They give you refills before the show starts. After the show, you can stay right there for a good view of the fireworks and they'll refill your drinks again. Some people call it highway robbery. Some people say it's very worth it. We loved it, but it was our first time to DL.
For the 10:30 show, a good method is to hang out in Frontierland and Critter country, ride HM, Pirates, BTMRR, etc. then swoop in after the crowds leave. After F! and the fireworks, the majority will be headed out of the park, and you will be following on the back of the crowds to River of America area. I have heard that it is pretty easy to walk up to a front row seat and only have a 30-45 minute wait.
We only did it the once, with reserved seating, but it was a great show. I think we'll do it again in Oct. Hope some others chime in here with more ideas on how to see it!
Hope that helps!
