Derailleur Pulley/Jockey Wheels – Check that the teeth of your derailleur pulley are in good condition. If they are sharp and pointy, consider replacing the pulley wheels. If they're full of gunk and grime, scrub and wash each pulley with a degreaser and brush until clean. And remember, lube is best applied to the chain rollers, not all over the pulley wheels, where it will attract gunk.
Rear Derailleur - The rear derailleur is a key component when it comes to shifting performance and a good day on the bike. Check the shifting with your bike in a stand or have a friend help hold the bike with the back wheel off the ground. While turning the pedals, shift through the full cassette.