Can anyone suggest a solution to this problem? I have tried the following in the last couple of days: tightening the seatpost nut (a little), cleaning both the seatpost and the plastic bushing with isopropyl, partial wrapping of the seatpost with packing tape where it contacts the plastic bushing.

Seatpost still slips... note that I weigh 72 kgs/158 lbs. The bike is approximately 2 weeks old... is there a break-in period for the seatpost? Should I get a replacement plastic bushing?