Now my favourite general method is this.
I like to have everyone in the fortress, except miners, be in the military, and I do this ASAP. There are a few melee squads, each a squad of 2, and everyone else is in squads of 10 and are crossbow dwarfs. The melee squads are always active and train permanently at strategically positioned armor stands. The marksdwarf squads are always inactive and have no assigned place to train. Now to avoid complaining about the draft, it's important to get at least one military level. The easiest way to do this, is to get the marksdwarf squads to shoot bunnies or elephants or whatever other wildlife rears it's ugly head. Live target practise enjoys 3x the experience per bolt. Also it's nice that marksdwarves can't hit anything until they are at least novice, so if you get a bunch of nubbies to shoot at a fox, the fox will actually live until they can actually start hitting it somewhat reliably, but all those misses still give experience, so it's very target-efficient to train up from dabbling to average-ish on wildlife. Once the dwarves have a military skill they no longer get upset about being drafted, and as long as they also have a civilian skill they won't be upset about being released from duty either (watch out for peasants). When an inactive squad is stationed, the dwarves spring into action, and once it's un-stationed, they return to civilians.
I should add that the melee dwarves are metal-clad, they function as meat shields and executioners (since crossbows are wonderful for disabling enemies, but terrible for killing them), no expenses are spared for the melee dwarves and since they are few in number even on maps without iron or flux they can be steel clad. Often two of my starting seven will be the first melee squad, which gives them time to become elite before sieges start.
The marksdwarf squads are armed with just this: crossbow, shield, cloak, hood. This is mainly to keep the weight down while they are doing their normal civvy duties. If the fortress is prosperous with much metal, I upgrade their armor to: Helm, Mail Shirt, Gauntlets, High Boots, Cloak, Helm. This provides total metal coverage while still keeping weight down.
The advantages of having everyone armed and armoured are many. Inactive military dwarves are frightened of nothing and do not hesitate to use their weapons against intruders. One of my funniest dorf moments ever was when Urist mcDumbass went moody, now he insisted on some of kind generic boring grey rock which I didn't have. Lets say it was chert. I had diorite and granite and any number of other generic boring grey rocks, but Urist wanted CHERT to make his CHERT STUDDED LOIN CLOTH! Eventually, with a howl of berserk rage he rose to his feet, instantly a bolt struck him between the eyes and he dropped dead with a surprised look on his face. His reign of berserking terror lasted 1 tick and cost 1 bolt.
When a siege turns up, everyone fights (except the miners). If you're feeling wussy, you can make a special squad for your legendary armorsmith and brewer and other essential dwaves and keep them inside during sieges to protect them from snipers - but still keep them armed and armoured because dwarves love to sneak outside when you least expect it.