Sometimes when I get good democracy going, I'll find a city with 20+ free shields, get it celebrating WLTxD and designate it a settler city (kind of like a wonder city or a SSC). I'll set a few settlers (or engineers) aside for just building infrastructure roads to new city spots and every 2 turns, my settler turns out a new settler which simply goes to the ready-made spot and founds a city. This will churn out cities pretty fast, as long as you have free space to build. If the city has 10-19 free shields you can partially buy a unit to 30 shields after 1 turn to get the settler the second turn. Later in the game if you want go all out ICS, you can take several 40-shield cities and try it

Usually helps to have a military city as well (1 turn defender units) and you have mass-produced, defended modular cities

This method also works if you are just really lacking in settlers. build a settler, re-home it. repeat.
*thinks his ICS roots are showing*