I generally use military as my exclusive defense (no walling in (at least one three-wide opening), no traps, and no danger room).
My method is to have at least two, sometimes three, melee squads for every one ranged squad (except at first, my first squad is melee and my second is ranged, after that I try to stick to the ratio). I try to have blunt weapons, axes and spears (for reasons listed in posts above) and usually have groups of ten and have them set to train every other month.
I set my courtyard (partly walled in area on the surface) so enemies will come through the front gates and clash with my melee squads while my archers rain arrows down from above, seems to work pretty well.
I also use war dogs; as others mentioned they aren't great in a fight but they add additional targets and sometimes get in a lucky hit. I pasture them at my gate entrance so they aren't assigned to any dwarf (thus no sad thoughts if they die). When the enemy hits they'll naturally attack a hostile creature nearby and during the regular shift they are good for spotting goblins and other thieves who try to sneak in.
I generally micro-manage two or three military squads and the rest I just set up with a basic uniform set and leave them alone, they serve as fodder for most battles. For my main squads I try to keep them updated with ideal weapons/armor (aside from what you can make sometimes you find better weapons from enemies or can buy them, then I just assign the specific weapon/armor to the recruit I want). I also try to keep an eye on traits (fast healing, tough, etc) for dwarves I pick for those squads. My 'infantry' squads (the non-microed ones) are generally just superfluous dwarves from later migrant pools. When in combat they get to go up front to absorb the initial attacks and distract the enemy while my main squad attacks. Its cold, but when you don't use traps, turtleing, or danger rooms you need every advantage you can get.
As far as materials go, steel is ideal of course, and in general silver is better for blunt iron for bladed, etc, I'm sure the wiki is full of details. I just go with what I have. Early on it may be copper, even bone, but anything is better then nothing. In my fort right now I was lucky enough to find iron and will get that going as soon as I get my basics up and running, but in the fort before that (eaten by nightwings..) I had to make do with less for a while.