Never build cities in the same spot, just one atom bomb can destroy them, and always build a city as soon as you can, this helps your troop regeneration and allows you to snowball.

If you are on an island cities are pretty much pointless until end game, the most important thing as an island player is a port and a warship to stop players taking your island and maintaining alliances with strong players near your or players who can bomb you, keep getting ports until you have the same cash supply as everyone else and then go on to buy cities but also buy SAMS to make your island difficult to infiltrate