My favorite parts from the official announcement:

Try to avoid interstitial pages, ad pop-ups, or other elements that interfere with your content. In some cases, these distracting elements may be picked up in the preview of your page, making the screenshots less attractive.

and

Currently, some videos or Flash content in previews appear as a “puzzle piece” icon or a black square. We’re working on rendering these rich content types accurately.

From here on in, I’m an Instant Preview Optimization Specialist™ (IPOS)