The only time I fight prolonged wars is when someone everyone likes declares war on
me; I'm not going to pass up the opportunity to wipe them out!
Otherwise you get those stiff diplomatic penalties. If
they declare war though, regardless of how many times they sue for peace or how many cities you raze, the other civs won't lift a finger
One thing I do is make sure to get the Pyramids with whatever industrious leader I'm playing as, and use the Oracle to get Civil Service. It's still possible on Monarch, not sure about the higher level handicaps. Bureaucracy + Confucianism + Courthouses + Macemen = you win
I go to war after I've got a solid foundation in my cities with forges and troop production is going strong. If it's a 2-person continent I usually have Cho'Ko'Nu (with Qin) before they get Longbows, and am 5-6 techs ahead with a strong infrastructure. Once they get Longbowmen (or horse archers) I add in Macemen.
Dh_epic's points are
excellent for an early continent-takeover! Only keep the best cities (razing all the outposts), disband units when you're done, and make sure to get Optics when you can. It'll allow you to contact the other continent and get that +1 movement bonus, plus it allows you to trade your map and techs for their techs. Always beeline for Paper -> Education, and never trade it away. Universities can give you a
huge advantage over the AI, and they give Paper a very, very low priority if someone else has already researched it (and done the initial map trading). Spread your religion and those you capture to all your cities, and build Monestaries for the +10% science research each (they're also dirt cheap).
Unless you're going for a cultural win, skip less useful techs like Music and Drama. Only research Archery or Horseback Riding if you really need it (usually axemen suffice), and skip the Calendar until you want your cities to grow past the happiness limit. If you get the Pyramids that'll practically eliminate unhappiness in your top 5 cities. The Great Library will significantly improve your science output (doubled with Representation), and the Colossus and Great Lighthouse will dramatically counteract your maintenance costs if you have lot of coastal cities.