[Phil]

AL:
Very true on heavy or large surface pieces. Also it is necessary to develop a proceedure for thin materials such as when fabbing a tank or welding thin tubing.
Open edges need to be welded first, then work towards them as they will melt away if you just run the bead towards them from a heavier spot.
I usually pre-heat larger pieces to get a uniform weld, and to cut down on current requirements at startup. It is not unusual to start at 175-200 amps and wind up with too much heat even at 100, the material really soaks up heat. It may be necessary to cool the part and go back to it to avoid the excessive heat build up.
Also be careful when handling a really hot piece as gloves can introduce nasty stains in the surface that are hard to remove.
Pre-heat is particularly useful when welding a thin part to a thicker one, until both are close to equally hot.
As Brian mentioned there is a nice helium/argon mix that works well, generally I use straight argon unless the job has certain requirements or I need a particularly nice looking part.
It may also be advantageous to back purge small tanks as you build them to avoid snots and porosity on the inside, this can be done right off your existing gas flow with a Y adapter and a small valve.
Get some scrap pieces and try your hand at different weld configurations to get a feel for the way the material behaves. Positioning means a lot, and you will find that you will get a preference for weld position and direction to produce the best work.
Hope some of this helps
Phil