I ordered a bunch of aluminum bars and cut them into 6" pieces to practice basic welding joints. It's going pretty well. Here's a list of quick tips that I have learned:
1/16" electrods seem good enough to handle almost any task at 150A or less. I'm not yet sure why anyone would use anything bigger or smaller for less than 150A.
The filler rod must be forcibly pushed into the puddle, then quickly retracted. It cannot be brought near the arc slowly, as it will melt before it gets to the puddle! I have found that I move the torch in the direction of the weld to get the base metal molten, then move it backwards slightly while I add filler, then move forwards again. I use this one-step-back, two-steps-forward approach for the entire weld.
The AC balance control can be pretty heavily shifted to DC EN. I usually keep mine at %70 to %80.
It helps to have the part propped up off the table. For the first T-joint that I welded, I had the plat laying flat on my metal table. This makes the angle of the torch more difficult, but worse, it sucks a lot of heat out of the aluminum that is touching the table. This is really a pain, because it makes the weld asymmetric in terms of heat required, and finding the magic torch angle to melt both sides evenly becomes more difficult.