WD40 helps as well. Also depending on what you are getting a snipe to slide over the breaker bar helps. If I have a lot of tools with me I will leave them in the car, go scout out what I need(its a big yard) and when I've found what I need I'll go back and get my tools.
DO NOT let your tools out of your site. People will steal them.
You are not allowed to bring jacks in or any kind of flame(ie propane torch).
If the yard is muddy/snowy pull the carpet from the truck of a vehicle to lay on. Bring some coveralls so you can peel them off after and not get your car dirty on the way home. I use latex all the time when working on cars now and its handy at the wrecker as you just peel them off when your done and yor hands are clean.
Core charges apply on many parts so if its a big item(diff,tranny,etc) you may want to bring the old one with you for exchange.
Oh yeah, its not your car, its going to be wrecked anyway, don't worry about being *nice* when pulling it apart.......