My favorite defense to multi

Direct Seat:

(2D)-?
    Pass = not done with good hands
    Double = 15+ balanced or semibalanced, or 19+ 
	System on in response; 2M to play.
    2H = takeout of spades, 11-18, nf
    2S = takeout of hearts, 11-18, nf
    2NT = either a strong jump overcall in a minor
          or a good 5-5 with a major and a minor.
	  3M is forcing.  Advancer usually bids 3C
	  unless he wants to bid more over a strong
	  3C overcall.  All weak hands bid 3C.
    3C = natural overcall
    3D = natural overcall
         3m is limited by failure to bid 2NT.  If
         their multi has many strong hands, we play
         3m as primarily preemptive.  If 2D has at
         most one strong hand (22-24 balanced, for
	 example) then 3m is a normal 3-level overcall
    3H = natural, sound
    3S = natural, sound
    3NT = natural
    4x = Namyats

Pass followed by double is penalty, 
usually a hand with both majors 13-15 ish,
but sometimes a so-so single suiter in a 
major that turned out to be their suit.
Pass followed by bidding shows a light action.

Advancing Seat:

(2D)-Pass-(xM)-?
     
    All actions are identical to direct
    actions over an xM opening bid.
   
Doubles by partner of a doubler are penatly.

Most of this is from Mike Shuster, with tinkers
from many others.