1. Clean the chain.
Most wet chain lubes double as excellent degreasers. Cover the chain in the lube, grip the lower segment of the chain with a towel and pedal backward. Keep scrubbing until the chain sparkles.
2. Clean the cassette.
Take the rear wheel off the bike and squirt lube on the cassette. Floss between the cogs with a towel to remove the tenacious black grease.
3. Center the brakes.
Loosen the 5mm bolt securing the brake caliper to the frame and reposition the caliper so the brake pads are roughly evenly spaced on either side of the rim. Retighten the bolt. Most calipers have a micro-adjuster bolt on top of the brake arm. Use a wrench to screw this bolt in or out to finely adjust the pad position until they are evenly spaced on either side of the rim.
4. Adjust the rear derailleur cable tension.
Put the chain into the smallest (hardest) cog and upshift once. Look down from above the cassette to see if the upper derailleur pulley lines up with the second cog. If it is closer to the wheel, turn the barrel adjuster—the black plastic piece the housing inserts into—clockwise. If the pulley is further from the wheel, turn the barrel adjuster counter-clockwise.
5. Check the bolt torque.
Even a properly tensioned bolt can loosen itself over time. Use a torque wrench to check every bolt on the bike, paying particular attention to the stem bolts.