For thickening the filling in fruit pies, Chowhounds think Minute Tapioca works better than flour or cornstarch. TrishUntrapped likes the way it keeps the fruit's juices clear, and finds it more reliable than cornstarch. magiesmom grinds it to a powder in a coffee grinder before mixing it into pie filling, and says this helps it dissolve more evenly.
Several hounds also recommend Instant ClearJel (a type of modified cornstarch) for fruit pies. It's flavorless, very stable, and thickens "perfectly," Becca Porter says. MsMaryMc agrees: It "works better than any other thickener I've tried!"
Discuss: Pie thickener
Photograph of Mixed Berry and Anise Pie by Chris Rochelle / CHOW.com