My policy is to sell between when MacRumors reports on the rumored date (confirmed by WSJ, Jim D., etc.) and when Apple actually sends out the invites. The last few years, the mainstream press and local news has even reported "New iPhone expected to be unveiled by Apple on September ___." When people see it on the news, that's when the value starts to drop.
I remember I sold my iPhone 4 for $580 right before the 4S came out. That is the best I have ever done... considering it sold for $650 unsubsidized new. The person who bought it had to be clueless and had to have been kicking themselves once the new phone was announced.