I work with LN2 all the time in my superconductivity research group at Washington University and I just wanted to say a few words.
There is no need for pre-cooling the container with another substance prior to the addition of LN2. LN2 has an extremely low boiling point (obviously), and, in small amounts LN2 will vaporize imediately when in contact with a higher temperature system. The temperature gradient upon addition of small amounts of LN2 will only exist at the boundry between the container and the LN2. It will not penetrate more than a few microns into the container itself. If you want a demonstration (a good way to startle your freinds who have little knowledge of LN2), put some in a bowl and splash a little on them with your hand. You will notice that your hand is is not cold at all.
Initially pour the LN2 SLOWLY into the container. Here your main goal is cooling the container. There should be a nice fog that spills over the sides. Don't pour so fast that the fog shoots noticeably upward. When you observe that the fog has substantially died down, your container is adequately cooled and you can freely pour the LN2.