Locking the suspension

How do you lock the suspension for when you want to change a tire or change the brakes or something like that?

You don't.  You can probably block it to prevent it from lowering but unless you disturb one of the height sensors (Disco) it won't lower..  usually.  There is a company who makes a manual way to inflate the suspension that would work I guess.  

so how do you change your tire without risking messing up the suspension?

in a p38 you leave the tailgate open to lock the suspension. lift the truck under the axle to change a tire/brakes

Does that almost work when I bring it a mechanic and they want to put it on a lift machine?

in the lr3 . i remember just putting it in offroad height .then holding the switch untill you hear a chime...i believe a light will come on in the dash.
after fighting the air suspension for 20 min,  i had to do what no man should have to do to change a tire...USE THE MANUAL !

tell them to use floor jacks and not the lift. locking the suspension only works when the vehicle is on the ground.

If you are worried about picking up a D2 with SLS, don't worry.
I lift mine all the time, sometimes the suspension will settle sometimes not, but as soon as you put it back on the ground it will reair back up.