When you arrive at the park, look at the map. The route followed by the parade will be prominently marked. The parade usually winds from the front of the park, in Town Square, to the back left corner between Frontierland and Adventureland, exiting right by Splash Mountain.
In general, the closer you are to the back of the park, the easier it is to find a spot. Most people seem to cluster at the front.
One caveat - I do tend to avoid the parks when it's busy, so this may not be valid all the time - but every time I've tried, I've walked up and gotten an excellent viewing spot in Fronteirland just a few minutes before the parade passed by.
Here's another tip for fireworks viewing if you're interested. For Wishes, the best view integrating all aspects of the show is obviously from Main Street. But that's where everyone goes. If you're interested in a different view and don't want to stand among all those people, there's an excellent viewing spot on the bridge between the hub and Tomorrowland. Specifically, there's a spot on the bridge just on the Tomorrowland side of the center point from which the view of the castle is obstructed by a tree. Nobody ever stands there because they think you need to see the castle to enjoy the show. You can always walk up minutes before the show starts and get a spot right along the railing. What people don't realize until the show starts is that from that angle, the fireworks mostly appear to the right of the castle, so while you can't see the castle from there, you get an unobstructed view of most of the pyro. As a bonus, Tinkerbell flies almost right over your head.
Note that while you can get a good spot right up to when the show starts, you should make sure to arrive before it actually does start, because once it does all the people who think they have a good spot because they can see the castle realize that the tree is blocking the pyro, and many of them will shift into this area. You want to already be there before this happens.