I view plan Bs as parachutes and won't go on a mission without one. Usually, the best plan Bs are centered around the getaway. I always try to set up some hidden mines (nonlethal/gas) in an alley near the target location. I try to have a secondary getaway vehicle at the ready (usually a stolen one). I scout out possible getaway routes and if possible I prepare some nasty surprises for following vehicles. Of course, an emergency hideaway is a must.
The real pain in the butt is always that dang plan A. I usually try to cut the endless "what if" discussions short at one point, as no plan survives enemy contact anyway.
Be prepared. Bring a gun.
