1) Bring along enough units to take and defend a beachhead at the very least. The number of units may surprise you. Often you may need at least 60 units to get the job done.
2) If possible, bring along a great artist to culture bomb your beachhead city to immediately bring it out of revolt. This will help with city/culture defense for your units when counter-attacked and also give you some cultural room to breathe.
3) Try to prioritize researching flight for airports. When your beachhead city comes out of revolt, rushbuy or whip an airport while you also rushbuy/whip airports in as many cities as possible back home. This allows you to airdrop many units per turn to your beachhead city and allows you to more easily conquer outwards without waiting for your ferries to get back and forth. Combine with 2) for greater effectiveness.
4) As always, try to hit the capital/big cities first to cripple your opponent. Coastal capitals should be prioritized.
5) Yes, if you can strike from two points at once this can be a powerful move. However, remember "divide and conquer" as well. Make sure at least one of your stacks, and preferably both, is sufficient to get their job done.
6) I find having a tech advantage helps a lot, especially if you are fighting before airports and an airforce. Fighting at tech parity (e.g., early renaissance) increases the numbers of units you need to be bringing along and can make it harder to wait for your ferries to bring reinforcements. If your first stack does not succeed in taking and holding a beachhead, your war just became much, much more difficult.
7) Try and consolidate your own continent back home so that you can speed things up overseas by taking vassals. If you are able to vassalize an opponent this gives you the peace and time necessary to ferry in reinforcements and to prepare for your next assault.
EDIT: 8) Make sure to bring along enough warships for naval stack defense and for bombarding coastal city defenses. Your land-based siege are not useful for bombarding coastal cities.
9) One thing I like is to bring along marines, if at that stage of the game, and to have them go with my navy to sack coastal cities (since they have amphibious promos) while my land-based, city-raider army (usually tanks/bombers at that stage of the game) proceeds inland.
10) Another thing I try to do from time to time is to bring along a couple aircraft carriers with (jet) fighters. This air support can be helpful for collateral damage, bombarding cities, stack defense, and so forth.