One trick that I like to use is to "clump" my nukes so that they are coming in to strike at about the same time. Even if your enemy has a good anti-air defense, they can only shoot at so many nukes at once.
To do this takes a bit of practice, and some timing: you need to launch from the further-away launchers first, then launch with the closer ones so that the missles end up being very close to each other once they're close to the target. With a lot of practice, and some imbalance in your initial placement of sea forces, you can get you 6 ground launchers, 12 subs, and 18 bombers (6 ground, 12 ship) to all release nukes in a cluster (for a total of 36 nukes), and maybe loose 4 or 5 (less, if thier defense is split).
Remember: you only need them to be in-sync near the target's defenses -- once you're past those, they can (and should) go to various targets.
This is all much easier if you make liberal use of the really-really-slow button. Just make sure you speed it up whenever you're not needing to do something, lest you piss of the other players.