Right now the best way is take a shuttle bus (Line No. 1) or a taxi. Allocate at least 1.5 hours. Refer to ShanghaiAirport for updated info.
Route: Airport bus Line No.1 Tel: 8621-68346612
From Pudong PVG Airport To Hongqiao SHA Airport (vice versa)
Terminal 1:7:00~23:00 (flight over)
Terminal 2:7:05~23:05(flight over)
Interval (Mins.) 15-25
Fare (RMB) 30
In the future (after May1st 2010, according to Wang Jianshuo) there is the Metro 2, which will link those two airports.