I use the old orange handle "ice pick" that Byron used to include in their kits. Heat it up by laying the tip on a monokote iron set at 350 degrees, push it into the end of the tubing and leave it there for 10 seconds. After putting the tubing over the nipple I wait at least 5 minutes for the tubing to cool down and then safety wire everything.

For larger tubing I use an awl the same way.