I find the more often I change the nectar the easier it is to maintain the feeder. You won't be battling any black mold and you'll have a lot more hummingbirds and orioles.
I usually just rinse the feeder with hot water. To clean the hard to reach places you can use a pipe cleaner or we have special little brushes for cleaning feeders at Wild Birds Unlimited.
Now that we are in the hot and humid weather, you should soak the hummingbird feeder for about 5 minutes in a Scoot, active enzyme cleaner once a week. Or use a one part vinegar to nine parts water solution to clean the feeders. Then rinse your feeders thoroughly.
Nectar (sugar water) recipe
1/4 cup white granulated sugar
1 cup water