Aluminum Welding Q's.... on my boat

[Permafrost]

I wasn't sure where to post this, but off-topic seems about right?

I'm in the need of some input, tips and tricks on aluminum welding from you welding gurus...

I'm modifying an aluminum boat hull. Building up the transom and adding some bracing. I have plenty of experience MIG welding steel, but aluminum is a different animal.
I have a Miller Regency 250 powering a Miller spool gun, aluminum filler wire and argon shielding gas.

I know to push the weld with aluminum rather than pulling with steel, and really have to move, but my welds are not very pretty
I can lay down a pretty decent bead after I get rolling, just starting and stopping is challenging.

Any tips for a novice? Besides going TIG...