You can use just about anything.
My chair has hens & chickens in it, & requires little or no care.
Then I have a rocker painted in rainbow colors with a fuchia on the seat. My goal is to plant a peony under it so that it will have the rocker bottom as support & maybe not fall over so bad.

If your chair still has the seat part in it, you can sit your pot on top of it or you can cut a hole in it to set your pot into. If you have a chair without a seat, you can staple chicken wire to it and line it with sphagnum moss or a cocoa liner and plant away. Here are a couple of links that give instructions.

http://www.frugal-home-decor.com/articles/042701c.shtml

http://www.thriftyfun.com/tf28249398.tip.html

Here is a pic of one of my chairs that I put a cocoa liner in.
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And this one has an old dish drainer that I stuck down into the chair to set a pot into.
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Just use your imagination and you can come up with some pretty chair planters. As far as which plants to use, most any plant will do well. Please post a picture of your chairs when you get them finished. Also, check out the Garden Junk forum. Some people there have made plenty of beautiful chair planters as well as other things you might be interested in. http://forums2.gardenweb.com/forums/junk/

Here is a link that might be useful: Shysue's Album

Shysue, I love your petunia chair!
Check the link below to see another chair. It's a blog site of a lady that has a gorgeous cottage home. Scroll down to Aug. 6th. I check her blog almost every day.
Debbie in Florida

Here is a link that might be useful: Cherry Hill Cottage