The obvious answer is auto-calc. :(

If you're following the dread path, use spies extensively, three per conquest will almost certainly get you over 100% chance to pop the gates.

Don't fight in the city square. The enemy has effectively infinite morale standing there, so force him out of there before you engage. A single unit of artillery is usually enough to drive him out, or at least reduce him sufficiently that a good charge will clear most of the remaining troops away.

Attack along multiple angles, as others have described very well.

Don't fight in the gates or choke points, also handled well elsewhere in the thread.

Finally, if you have high quality troops or trust your tactical sense, incite the AI to sally out against you by bringing an apparently inferior force to the siege. You should win 99% of the AI's sallies just by charging him immediately and trapping his men in the gates.