First, get some viscoelastic speaker cabinet damping materials such as Soundcoat or Deflex. Apply them to the insides of the cabinet and to the entire interior surface of the removeable back if possible. "Green Glue" is a special sound deadening adhesive that's ideal for this. Then, cover all the interior surfaces with convoluted (eggcrate) foam, the type you use for a mattress pad.

Next, damp the speaker basket with thick beads of electrician's putty, which is available at all electrical contractor's supply houses and most of your large hardware stores.

Make sure the cabinet is airtight. Caulk the inside joints and where the wires enter if necessary, and place a thin gasket around the mating surface of the back panel if needed.

Finally, if there's any way to add some 2" x 2" braces in the cabinet (especially to the removeable back panel), don't hesitate to do it.

I learned these simple tricks over 15 years ago from a fellow player. After applying them to my 4-12 Marshall cab, I was blown away by the improvement! The bass is much tighter and drier, with all the tubbiness gone. The dissonant cabinet resonances and speaker frame ringings are eliminated, and the mids and highs are much clearer.

These tweaks are standard practice for hi-fi speakers, but I'd never thought of applying them to a guitar speaker before my frind told me he did it with all his guitar cabs. I'd say the improvement is even more dramatic with a guitar cab than with a hi-fi speaker!

Thermionic