I went cheap as chips with my preparation:
Flashing tape: £15 a roll (screwfix) x 2
Closed cell foam: £45 for the lot (ebay)
Carpet underlay: £15 - carpet shop
Panels deadened, noise canceled and higher frequencies absorbed from bouncing about.
Works rather well.
Oh, also have some sound paste / seam sealer type stuff for little gaps / hard to reach place and inside of speaker pods / sub enclclosure.
Good stuff my mate ;)
Got a link for that sound paste magic stuff? ;)