When my buddy, who had a hobby as a reptile breeder, decided he needed generator power backup in a rented house, he chose to bring emergency power into the main breaker panel using the popular but unsanctioned method of a cheater cord into the dryer outlet.

The reptiles needed warmth to stay alive, and his collection was worth tens of thousands.  Because he had several rooms full of cords feeding low wattage tank heaters, and also wanted to supply the usual refridgerator and furnace loads, the extension cord method was a real nightmare, would require a big investment in cords and splitters, and hours to setup and break down.

Dryer Outlets are attractive for this purpose because they are high rated, fed via large gauge wire and generally located close to the service panel.   My friend's house was equipped with an older 3 prong, 30A outlet.  I built him a cheater cord that came out of the generator and plugged into the dryer outlet, male connections on both ends.  This is unsafe if plugged into a live circuit on one end, and left unplugged on the other end, which is just one reason why the practice is unapproved, but he wasn't interested in spending a lot of money upgrading the landlord's property, with a proper interlock, so took the expedient path.

It's essential to shut off the Mains, isolating the breaker box from the grid.  Also essential to shut off any 240 Volt loads, since they would be fed single phase during the emergency event.  Some generators can supply split phase 240V, but not the one he was using.  It is possible to make a cheater cord that will supply the same single phase to both legs of the split phase circuit breaker box, and this is a viable strategy if one is disciplined about isolation.

The last essential step is to identify which circuits are essential, such that the generator, dryer outlet, and inlet cord won't be overwhelmed.  We used colored tape to ID circuits which must be OFF in red, and those to turn ON in blue.  Anything left unmarked was safe to turn on, but not essential.

It's a practice that is a half measure, and not by any means idiot proof or without risk.  But, it does work, and requires no modification to a building's electrical system to implement.  The only evidence of shennanigans is that cheater cord, which can be quite short.

Axmann88:

You covered the "cheater" method well, and I did something very similar when we had a power outage that lasted almost a week.  For the first few days we just ran extension cords to the things we needed - then when I found out the outage was going to be extended I went ahead a wired a special cord that went into the breaker box and supplied a full 220V supply from the 7,500 watt gas generator we had at that time.  It was essential that the "MAIN" breakers remained "OFF", and I was using a 30 amp plug on the generator that was wired into the 220V circuit breaker for the dryer.  Our house was running just like normal and all of our lights and TV and ceiling fans worked, and we could have run our 220V appliances - but we did not as it was summer and we didn't need a dryer, AC or to run the oven.  NOTE: It is essential that the person running the generator understands not to turn the "MAIN" breakers on while the generator is running....or when the generator is connected to the house circuit - and that only "ONE" person operates the setup so he knows exactly what is on or off and knows to unhook the generator before turning the "MAIN" back on.  I feel that any person doing this needs a good understanding of how electricity works and travels though a wiring system.  When you make a cheater cable to allow this method to be used - those live terminals poking out of the plug need to treated with respect!!!!

MMRanch:

One easy thing you could do to avoid the emergency cords, is to install some new wiring in your house that is dedicated to the generator.  You could have an outdoor plug....or just a wire with the proper 30 amp plug and and some 10 gauge wire to a small breaker box.  You could install a couple of 20 amp  breakers and run some 12 gauge wire (or 15 amp breakers with 14 gauge wire) to the places where you plug in your fridge, freezer, TV and a few lights that you want to run when the grid is offline.  

This would be pretty inexpensive to do....and horribly convenient for you.  When the power goes off you move your generator to the place you have the outdoor connection and plug it in, then start the generator.  Then go inside and unplug the fridge from the original outlet and plug it into the new generator outlet.....repeat as necessary for the outlets you have installed.  (NOTE:  Be careful with any electronic equipment - have it unplugged when you start or stop the generator).

I could help you do this work next summer if you want.....I could ride a motorcycle down and we could combine the work and play for a few days.