It can increase endurance, toughness, and strength, so I always give my soldiers-in-training a long tour of the pumps. Exhaustion is the #1 cause of dwarf failure. A dwarf in full steel will eventually fall when exhaustion overtakes them and they collapse. Then, I usually give my dwarves a shield and weapon for standard recruits, or a pair of shields for the elite guard, both with full armor. Either way, they're then fed live training from usually captured sieges, sometimes from creative wild animal breeding projects and distribution boxes. Once sufficiently high ranked, they're entered into back-line military duty and work their way towards front line. Standard recruits usually get ~5 in shield/armor/dodge and ~7 in weapon before I pull them, usually using wooden training weapons if available. Elite guard get towards 7 in shield/armor/dodge and up wards 3-6 in misc object, wrestling, and striking, depending on how the individual dwarves fight, before being given a weapon and put into active siege duty around mid-row.
Using numerous burrows, I line up my soldiers in formation inside a large "killing field" designed to contain and keep the sieges somewhat orderly, essentially a massive open air arena with creative locks and whatnot to ensure no escape while also allowing the entire siege to become trapped before the troops are set out. Elite axedwarves are in front row, elite speardwarves and hammerdwarves directly behind, elite-in-training next, regulars-in-training after that, and finally some hodge-podge marksdwarves in the rear who mainly stay in place and provide additional flying chaos that doesn't contribute much.
In case of whips, I'm usually able to flood the combat zone, or more likely capture them, retrieve the weapons, and used the still-armored goblins as training fodder.