The battery trick helps as well - connect a 1.5V battery to the driver. First to push the cone outwards so it'll be easier to apply the glue, then to pull it inwards which helps to center the cone. I used this with the method from the Good Hifi manual with good success, although I had to redo one of the six drivers from my CX50+100 project (check for scraping before the glue sets completely so you can still pull the outer edge off and try again).
The cardboard shell of a Bowmore bottle can be cut into rings that are the perfect size for weighing down the outer rim