Start here ..
Personally I started with the X Axis stiffening mod using bolts between the 2 maker slides, however I decided to go a bit more rigid and converted to 1 piece of extrusion as detailed in this post.
I've also make sure to stiffen the Y axis by attaching brackets.
Lastly, I upgraded my GT2 belts to GT3 belts and GT3 pulleys from OpenBuilds parts store. It made a big difference in the performance w\ aluminum and makes the machine behave as if it's carving thru butter now on harder woods.
Like anything else, YMMV, but I swear by the above mods. The only other suggestion I would make, not related to the mods, is make sure you attach the machine to a rigid surface that's dead flat. I built a torsion box specifically for mine, and I think that's eliminated most of the alignment problems that most others have right out of the gate.