You know the bank they use in the beginning?
Well, notice how it isn't in Liberty City? Or is it?
To do this you will need a dodo.
Go to the airport and highjack a dodo plane. Use the runway that goes off to the right.
If you circle around the city, personally, I find it much easier, and you will notice a mountian to your right as you circle Portland headed towards Stauton Island.
Fly in it. Dont be afraid. There is no berier to stop you. Now, after your in it, keep flying towards Shoreside Vale, still in the mountain.
Now head to the Observitory, still under the mountain. You should See the Bank, with a couple of roads that just drop off. Dont bother to land on it, 'cause you cant. The game will freeze.
If you miss it, just keep trying, it's there, okay?