How do I prevent the rear derailleur from slipping gears? What's the best way to tune the rear derailleur of my bike to stop slipping gears?  It seems that every time I re-adjust it, the middle/upper gears are perfectly aligned, but the lower gears never stay put.  When I'm climbing up hills, the chain keeps slipping up/down a gear, and never stays put.
If your derailleur is adjusted properly, and you are slipping gears, it's usually a worn chain and rear cog. Take the bike into your local bike shop, and they can help you get replacements.