There is no amount of planning that will aid a players choosing to have early wars. Cities were buffed to the point that anything less than longswords and catapults(preferably trebs) would be doomed to fail. Without a bonus to production players are not able to entertain the idea of successful early wars, it is a great option for those that don't want to but think about those that want the option of being aggressive and effective early on.
Urm.. No? I'm playing on Immortal now, almost exclusively huge maps on epic speed. I'm finding early wars still quite tenable, and not requiring longswords - though, requiring siege.
If you make a force of swordsmen with a few catapults and see a border city and say "Ok, I'm going to siege that because that's the city I want!" you'll be trounced by superior enemy unit mobility in their territory. But, one of the AI's greatest weaknesses is recovering from initial early losses in a war - once its unit wad is blown, it won't be able to counterattack or actively defend its cities successfully.
That being the case, my first objective starting an early war against an enemy which will have a unit advantage over me isn't to take a city, but to wipe out a big glut of units. Even if you're only running swordsmen and the enemy longswords, this is still quite possible through manipulation of choke points, terrain bonuses, etc etc. Find a spot near or in a poorly roaded section of enemy territory and set up your front line with some archers and cats and bring a worker or two for bait and start getting the enemy to move into position to attack your units. If there is a weaker border city - perhaps not the cit you want the most, but one which will offer you favourable terrain in which to fight - which you can take in a turn or two, grab that and wait for counterattacks.
I like going to war with two cats and two archers or three cats. If you end up stuck in a defensible siege, your objective should be to gain XP and kill the harassing units and just keep the siege going until you're getting 3 range catapults with two attacks and indirect fire - a long siege is a victory for the aggressor because of the existence of upgrades which will make your siege units *so* much more powerful. Your real objective should be to draw the bulk of the enemy's force into a field confrontation when you've got a nice ranged bombardment in a favorable position - because once you've wiped out that glut of units, you can move through the opponent's territory with only scattered token resistance.
I wage sword/cat wars quite frequently, and they can work with inferior numbers and tech - just make sure your first objective is destroying the glut of enemy units. Draw them into your territory, capture a weak 2-turn border city before they can react, set up in their territory in a defensive position and wait for the storm - whatever. Just get rid of those enemy units and then the opposing Civ will be ripe for conquest, since the AI doesn't know how to rebuild after a major loss of units like that. Also, one thing old Civ players have to remember about this game is, there's no building penalty for keeping a war going over the long term. If you've got a position set up where you're not losing much and just slowly racking up experience, you're doing great.