You've just spend a lot of money on a new bike, or you feel rather sentimentally attached to your old beater, either way you want to do everything you can to keep in in your possession.
Just a note, this won't work in an extremely high crime area, if you live in a place where people are willing to strip the parts off your bike, don't let it out of your sight. If your lucky enough to live in a city where you can just leave your bike unlocked, then you probably live in a city with more cows than people. For those moderate crime areas, this should work.
There seems to be a bit of a misunderstanding here. People keep on recommending better locks under the assumption that they are harder to break. Any lock can be broken, but if the thief has the time to take a car jack out and spend the ten minutes prying it open, its your fault for leaving your bike there.
People also suggest doing things like rounding off all the screws on your bike, or filling them with epoxy. Good idea until it breaks and you can't remove it.
The safest place for your bike is under your legs, short of that, by your side. At least within your sight. Locks, no matter how nice, will only help so much.
Step 1: Evaluate Your Bike
This is hard, take a step back and forget how much you love that hunk of steel or aluminum, and try to actually evaluate its worth.
If its not made out of steel or aluminum, its likely worth a lot.
triple butted anything, expensive
Carbon fiber anything, expensive
If it even thinks words like 'Campy' or Dura-Ace, its worth a lot.
Anything handmade (you don't count)
If your bike really is worth that much, and not just to you, the best and only way to ensure it doesn't get stolen is to never let it out of your site. Take it into your house with you at night and keep it next to your bed.
Otherwise you will probably be fine leaving it outside overnight.
Then again, if you couldn't bear to part with it, inside is always best.