I have an older racing 10 speed and i can't get it to shift to the smallest wheel on the back. I've tried adjusting the high gear adjusters and I think I made it worse. It used to slip between the two smaller wheels in the back, now it won't shift at all to the smallest one. Any suggestions?

My guess is the tab that the derailleur screws into (the derailleur hanger) is slightly bent. This is pretty common, and your bike shop will have a tool for realigning it. It will cause the exact symptoms you describe.

I'm betting that seely's right. Since this is an older bike, you might want to take a shot at fixing it yourself. This home remedy isn't as precise as using an alighment gauge, but I've had good luck with it.

Shift your bike into a gear that makes your derailleur hang straight down. Now stand your bike up against a wall so that it's perfectly vertical. Now look at your derailleur arm from the back of the bike. It should be exactly parallel with the plane of your main frame tubes. If it's bent inwardly a bit, that's your problem. Clamp a big crescent wrench onto your derailleur and gently bend the derailleur hanger back until it's straight. It doesn't take a lot of force, that's why they get bent so frequently. Resist the temptation to remove your derailleur so that you can clamp directly onto the derailleur hanger. If you do that, you'll probably distort the shape of the derailleur mounting hole.