If you have open borders... scout! See what they have already, lots of horse/ele's? Bring more spear/ele's to counter. Lots of xbows? Bring more mounted. You can also see what they can possibly build by looking for bronze/iron/horse/ele's on the map. If they only have one and its close to your borders go pillage it and camp the square for easy war of attrition win.
The thing i'm trying to do is war more. I only seem to do it when I want to invade all of the land, but early wars to steal workers, pillage a few tiles for gold (and to hurt the development of the other civ) which makes later wars easier. I think sometimes a civ's UU is more suited to pillaging/defense type roles, attacking units that are protecting resources or moving to attack your invasion stack. I plan to do it more to get more use from UU's that arn't city busters.
Quality vs Quantity? I use GG's for XP rather than +50% production. Units have upkeep/supply costs, quality does not.
Finally, whilst not a "war tip" I would like to point out that a good economy/building plan really helps. I used to rarely use the "Build Gold" or "Build Research" options and just stuck to buildings but now I see how much specialisation right down to Gold or Research helps. Even 10 hammers becomes 10 gold/beakers, a 10% building would only give more if it currently has 100 gold/beakers, nevermind the time to see the return for the multiple building turns lost. Linked to this is building gold in military cities, rather than pump out units that will soon be obsolete or you don't need yet, you can get quite a bit of gold by building it, this can then be used to upgrade units (or up the research if you want).