If you sell contact with a civ right before you destroy it, the civ you sold it to will be mad that you cheated them and will come to you next turn demanding a ransom payment (or else WAR!) Neato
If you are 1 turn away from building a wonder, you will always get it no matter how many other Civs also have only 1 turn left on it.
A city touching a source of fresh water (or an irrigated square) acts like an irrigate square itself, so you can run the irrigation straight through to the other side, even in squares not touching irrigation.
stacking workers to build roads and RR's is always more efficient than building one tile at a time for a couple reasons: first, each tile of road makes it quicker for the workers to get to the next. Second, if a worker pitches in to work on a road already being built, and his work finishes the project, you "reclaim" the turns of all those other workers working on it and get to move them on to the next tile in the same turn.
Remember the defender always gets a combat bonus! The absolute minimum is 10%. I believe this is why the computer seems to do so well despite units being evenly matched... as it is rarely the aggressor.
The absolute maximum defensive bonus that can be attained is 200%, which is awarded to any unit fortified in a fortress in a mountain across a river. The next best bonus is 150%, awarded to a fortified unit in a fortress in a hill across a river, or a fortified unit in a city (or town with walls) on a hill across a river.
An obvious one...make sure that no city ever grows (i.e. fills its food box) if a granary is about to be produced in that city, or if you are about to finish the pyramids. In most cases, you'll be better off going with zero growth for a few turns than letting the city increase before the granary is built, since if the growth occurs before the granary is built the food box will go empty (instead of half full). This saves a bit of growth time.
I saw a lot of (maybe old) posts telling you to build a city inside your rival's cultural borders. AFAIK this is not allowed in the latest version.
If your rivals are getting aggressive but you can't afford a war yet, try giving them Per-Turn payments of 1 gold. That way they won't declare war on you unless they REALLY want to...
Periodically giving them lump sums also keeps them from getting annoyed.. if you really want to play without beating them up