Don't lay out the whole plan at the start. Offer the first obstacle, plan how to overcome it, play through the plan. Then flashback to the planning table for the second obstacle, play through that. Repeat as needed.
If required, you might need one of the PCs to be carrying a suitcase or pack, which will have always held whatever they later decide they need for the obstacles in their flashback planning sessions.
This helps prevent wasting three hours planning, just to blow it all in one bad roll.
This trick gives the illusion of having planned brilliantly for many different contingencies without wasting hours and hours planning.
We did this in my Star Wars game tonight, and it worked really, really well. It really did give the feel of the well-planned scheme.
Plus, it dramatically reduced the amount of preparation I needed to do, because instead of having to come up with obstacles, and ways around them, and ways for the players to figure out the ways around, and all the details you need to game through finding that information, I could just come up with the obstacles and then let the players do all the rest of it. Beautiful!
If you are ever planning to run a break-in or covert op or heist-type scenario in an RPG, I heartily recommend this strategy!