What are the best sources of information for upcoming LEGO sets?
The LEGO store calendars are available online about a month ahead of time. It lists the upcoming promotions, specials, new sets, store events, etc. They are the exact same ones as what you would get in the mail. You can find them in the right-hand side-bar here: http://stores.lego.com/en-us/

As far as going as far ahead as the following year, you are not going to get official information on this, as LEGO doesn't reveal their future sets more than a few months ahead the most. You will have to follow rumors and leaks, which may or may not be correct. I would suggest that you become a member of either Eurobricks or Brickset and follow the discussions on possible upcoming sets. 

Also, it is a good idea to follow LEGO news at the beginning of the year, as new LEGO sets get revealed at the various toy-fairs in January/February. Pretty much all LEGO fansites will closely follow the news coming from these events. The sets displayed are usually go as far as the summer, but they will not include the large exclusives, which usually get revealed separately.