Keep the aluminum clean. Right before welding, clean off the invisible oxide layer with a clean stainless toothbrush dedicated to that purpose. Stop-drill the ends of tears. Clamp the joint carefully. When cracks persist, look for clamps that are too tight.
Tight clamps prevent shrinkage which, in turn, forces cracks to occur.
If misalignment occurs, or if the damps are wobbly or loose, snug them up. The sequence of skips can make a difference in crack resistance in some cases. Think in these terms of balancing the stresses as you go. End the weld properly. The crater that is left at the end of a weld as the metal cools, contracts and shrinks in upon itself is called a crater crack.