I'll start this off...
I discovered that I immediately got the highest spike in followers after Frank Kern sold out his Mass Control Monthly Membership.
What I did was posted that I made it in to reserve my membership with his system. You can see that here:
After I did that I went ahead and followed only about 100 of Franks followers ( I would've followed more, but I got lazy).
Next thing I know about 30 something people were following me.
Otherwise my previous experience was to get followers in drips and drabs.
IMHO since my post was extremely timely AND targeted, people who shared the same interest (i.e. Frank/Mass Control) people were much more likely to jump aboard than otherwise.
Anyone else have any other Twitter Tactics & Strategies?