I've never done a calendar spread. I just watched a Ryan Jones webinar where he talks about what he calls PPD (price per day) in determining which option expiry to go with. I might do some playing around with the concept over the w/e. Looks like a decent strategy if the SP continues to drop over the next few weeks.
http://www.optionpub.com/articles/?ID=29
http://www.optionpub.com/articles/?ID=29