I can't stress enough how important it is to OPEN THE MAIN BREAKER before you connect ANYTHING to your home wiring. You can energize lines outside your home and KILL SOMEONE.
While the pigtail method works, a dedicated transfer switch for specific loads is a safer method.
http://www.amazon.com/exec/obidos/tg/detail/-/B0000CCXUF/102-1523412-2314534?v=glance
In addition, many portable generators have GFCI's that get confused when connected to home wiring, leaving you with the requirement to run UNGROUNDED from your generator to the home panel.
Extension cords may be a pain in the butt, but at least they maintain the integrity of your home wiring's safety features.