We have found that adding terminals to the wire leads of the 4AA battery pack, which increases the surface area, works the best. To do this you will need a battery pack, wire stripper/terminal crimper, and 2 fork (or spade) terminals.
A properly crimped connection, which physically secures the wire and makes an electrical connection.
Optional, but highly recommended: Solder the terminals to improve physical and electrical strength of the connection.
Properly crimped and soldered terminals ready to be used.
We also crimp and solder terminals onto many of our Squishy Circuits components, such as this motor.