Two Words:
Overwhelming Superiority
Don't use 4 knights to attack a town defended by 2 spearmen, use 6 and have a few musket men for defense of the stack. Use 8 attacking pikemen...the idea being that you lose none of your knights as they retreat and once you take the city you can use it to heal quicker for the next advance.
Take a few knights and musket men (perhaps 2 knights/musket man) and ravage any advancing reinforcements. Knights move and pillage improvements and the musket men move up and defend them. Anything getting near a small stack like this can be attacked and then the knight can retreat back to the musket man for protection...these stacks are expendable to slow down the main counter by your enemy and are expendable to sever his connection to luxuries and SRs. That is their function...penetrate deep, cut supply and limp back home if possible...else draw off any defending troops into dealing with them directly.
If you are losing try for peace, you can look at it as a 20 turn cease fire...or fall back and defend what you can...the AI will attempt to maneuver around you, be prepared to conduct a fighting retreat to delay any advance.
Rush build as much as your population or treasury allow so that you can easily group reinforcements...you'll need them.
Lastly, always go after the AI with a specific goal, be prepared for targets of opportunity, but don't expect them, its much better to take one city and sue for peace (perhaps picking up another) than it is to overextend and lose your armies, the cities you've conquered, and face an enemy force in your territory.
