In order to make it 'legal' by means of back-feeding the panel (as most are describing above) you need something along these lines...
Available at:
http://www.interlockkit.com/
To be honest, unless you go all out and get an automatic switchover generator, I wouldn't do anything else...including the pricey panels which only provide like 6 or so circuits.
The aforementioned concept is really easy to install and works well. However keep in mind that many of the portable generators on the market are not suited for use with sensitive electronics. I know of many furnaces with fancy circuits boards being fried because the generator output is not clean enough.