Apologies if this already exists; couldn't find it on the forums, devlog or votelist.
Basically 90% of my hassles with my military (once I understood it and got it up and running) can be traced to the "Pickup Equipment" task. Either they haven't done it when I expected to already, or they wait to do it at the worst moment. Even using the 'Station' trick to get them to pick up their uniform doesn't prevent my marksdwarves from deciding, midbattle, that the -leather armor- a goblin just dropped is better than their current set, and they run outside to get it.
It seems currently the militia is very lackadaisical is equipping / upgrading their uniform, until you issue a squad order, when they suddenly decide it's the most important thing in the world. Typically when I issue a move / kill order, it's because there is something there that needs to be dealt with right now. I don't care if they have +Copper leggings+ while *Copper leggings* are in the stockpile, there's a troll in the dungeon!
I'm not sure how big a problem this is for other people, but it seems every other siege I'll notice a lone figure running out of the fort to upgrade to a better iron gauntlet left laying outside or something. It may happen more often since all my uniforms accept Partial Matches, but trying to be more specific is endlessly slow. The closest my fort came to !FUN! was all started when 2 recruits broke rank to go grab some cloaks outside during a siege.
So my suggestion would be to make "Pickup Equipment" an option of "Pickup Equpiment at will" / "Pickup Equipment when told" on the schedule screen, like sleeping is. Then add a "Update/Pickup Equipment" command to the squad screen. So, if you wanted, you could have your militia set to keep whatever they are currently wearing, until you issue the "Pickup Equipment" command, then they check to see if their uniform matches what they're wearing, and whether there is any available armor to go get. Or leave it at default as "Pickup Equipment at will." Basically a way to control when they do or don't choose to update their uniform, so it doesn't happen too late / when you least expect.