Kill yourself while holding the mad milk. When you respawn throw the mad milk and do not release left click. While holding down left click, pick up the mad milk you dropped earlier. You are now in the civilian pose.

Equip any Banner (preferably the concheror). Deal enough damage to fill the rage fully. Make sure you aren't in spawn, and change your secondary to the base jumper. Blow the banner and hold left click. While holding left click walk into the resupply cabinet. You are now in the civilian pose.

While playing as Soldier, equip and switch to the Beggar's Bazooka. Walk outside of your spawnroom and type kill in the Console. When you respawn, quickly go back to your dropped Beggar's Bazooka. Switch to it, and empty its ammo by holding down mouse one. The last rocket cannot be a misfire. While still holding down mouse one, pick up the dropped Rocket Launcher. You are now in the reference pose, signified by there being a bugged viewmodel.
This was found on the Civilian Pose TF2 Wiki Page

Equip any flamethrower other than the phlog (preferably the backburner) and kill yourself while holding it. When you respawn empty your ammo by airblasting when you reach 0 do not release right click and pick up your previous flamethrower. You are now in the civilian pose.

Equip the base jumper and any stickylauncher. Walk out of spawn and equip any shield (preferably the chargin' targe). Walk back into spawn and into the resupply cabinet while holding out the stickylauncher. You are now in the civilian pose.

EDIT: Something I forgot about: Under 'Advanced Settings' in the 'Miscellaneous' tab, "Automatically respawn after loadout changes in respawn zones" must be off for this to work.

Kill yourself while holding out your minigun. When you respawn empty your ammo on your minigun (preferably the huo-long heater) and when you reach 0 keep holding down left click. Pick up your old minigun. You are now in the civilian pose.

Kill yourself while holding the jarate. When you respawn throw the jarate and do not release left click. While holding down left click, pick up the jarate you dropped earlier. You are now in the civilian pose.

There is an all-class method that works only in MvM. It is described here:

• Walk out of spawn
  
• Equip a different weapon
  
• Walk into the upgrade station & taunt (while in the upgrade menu)
  
• Buy an upgrade while taunting & wait for the taunt to end

You are now in the civilian pose.

Credit to (っ● ‿ ●)っ for informing me of this method.

This section is a bonus section to thank you guys for all of your support and attention. This guide was meant to be a note to myself but also got over a thousand views so that's great!

Anyways, here is how to taunt with the contracker.

• Equip the contracker in your action slot
  
• Press your taunt key (Typically g)
  
• Press your contracker key (Typically F2)
  
• Press your menu key to return back to your game (Typically Esc) and spam the number of the taunt you want as you return to your game
  
• Blamo!

Notes: You must have a taunt equipped or else when you press your taunt key you will immediately do your weapon taunt. I have the Director's Vision, Conga, & Schadenfreude. Also, when I say "spam the number of the taunt you want", I mean the number in it's place on the taunt menu. Please comment with any issues.