Recently i've had a lot of success with the Great Wall/Great Spy gambit. Build the great wall, use the great spy on an opponent, and then steal everything they research until your points run out, usually around engineering or banking.
Possible downsides? The civ you infiltrate will suck and tech slow, especially if they get bogged down in war, and they might even be destroyed, though that's unlikely.
This works best on higher difficulties, on lower difficulties it's easy to outtech the AI from the beginning.