Characters and successful plans

Something I have noticed in many books is that characters don't really plan anything. The author will say that they spent time planning, but when you read what the characters are doing you realize they spent five minutes talking and wrote the mission down on the back of a bar napkin.
In fantasy novels this is seen most frequently when attacking a castle. The main characters will have two of their numbers dressing up as washer women trying to sneak in, while the thief will slink over the wall and "do something", and the big fighter will either hide in the washing, or pretend to be a fellow guard and kill anyone who gets in his way. For the most important part of the plan, if anyone is seen "ATTACK!"
This can be funny. It can even be a little thrilling, but for a reader who expects more then basic level strategy its extremely disappointing.
"Harry Potter and the Deathly Hallows" dealt with this nicely. So that I don't give anything away I'll be a little vague.
When Harry needed to get an item from a heavily guarded building, he and his friends planned for weeks to get in. When they got in they realize something. They screwed up. All of their plans involved entering the building. They didn't have a plan for if they were separated, if they were caught, or if their actions were discovered and security measures were activated. It took a bit of luck for them to get away, and Rowling did an excellent job of showing this without having to rely on deux ex machina.
Now you are probably thinking, but showing the characters planning everything is boring. You're right it can be boring, it can also be interesting but that takes effort. So here's the easy solution, don't show all the planning. Have them discussing some of the highlights, make it the equivalent of a traveling montage in a movie. The reader will realize whats going on, and it doesn't have to take up more than a few paragraphs.
Now why are plans important?
The main reason is most people hate deux ex machina. This means when everyone knows the hero should die, but something improbable happens saving his life, and possibly saving the entire day.
i.e.
The calvary just happens to be patrolling in that exact area at that exact time.
The hero suddenly develops a fantastic new power, saving everyone.
In at least one book, the God or his/her angels come down and whisk the heroes to safety.
I could keep going for several pages but you get the picture.
These are all tired and worn tropes. They can be done well, but it all too often comes out as poor writing.

So how do you solve this?
You plan the characters plan carefully.
This isn't the same as planning the plot of the book. You can still be spontaneous and write on the edge of your seat. But you have to ensure the characters have a valid plan.
If they want to kill a big evil character who goes to the same bar every night, and decide they will wait in the ally beside the bar until he comes out and stab him in the back, this is not a good plan. This is the equivalent of saying "There he is! Jump him!"
This can work. But you leave the reader wondering what about witnesses, what if he comes out with a friend, what if a gang sees the characters in the dark ally and decide to rob them. So many things are left to chance its ridiculous.

So what are some guidelines for planning your characters successful plans.
1) What information do the characters have. The more info they have, the better they can plan. When do the guards walk past a certain door in their rounds? How many people are likely to be inside? Who exactly is inside? All of these are critical to a successful plan. Just getting the information can be an adventure in itself.
2)What experiences do the characters have? A mild mannered accountant who has never even jaywalked is going to look at things differently then the hardened thug. But don't discount a character because they're inexperienced. The thug will have more experience, and know what to watch out for. But the accountant may take more time and effort planning everything out to the letter, so that he can get away without a confrontation.
3) What can possibly go wrong? This is wide open. Figure out what can go wrong with the plan. As the author you should know if Prince of the Realm is having a late night torture party which means the hero's can't climb through the sewers and come up through the dungeon to kidnap the princess in her bedroom.
4)How will the characters escape if something goes wrong? This is very rarely considered. Usually the author leaves it as a wild escape with the heroes fleeing the castle guards and having a few cool stunts. Or the evil person who catches them turns out to be an ally or not so evil after all, or has their own scheme that involves the characters. Again it can work, but it's been done to death.
5) Do the characters have the proper skills. Picking a lock is hard if you don't have experience. So when faced with the locked door that contains the McGuffin, does one of the characters suddenly remember they're past life as a skilled thief? Make sure before everything starts, that all the necessary skills and equipment are available. Write down a list of what their skills and equipment are. It will help.
6) Remember the consequences. If a plan is poorly thought out there will be consequences. If it is properly thought out there will be consequences.
If a plan doesn't work or barely works and the characters remain free what happened? Were the characters seen, were they recognized, did they have to kill a lot of people on the way out? All sorts of interesting things can come about from this. They might see their faces on wanted posters if they were only seen. If they are Rebels who were recognized in a bungled plan, they might become a laughing stock. If they killed people they suddenly have new enemies. Use these.
If it was successful, they might gain new allies who think the characters are competent enough to help. The enemy might put more of their resources into killing this now dangerous enemy. If it's discovered who stole the MacGuffin all types of new enemies might appear who want it, and know who has it. Again use these.

For further help with plans, watch some thrillers and spy movies. I'd recommend "Ronin" right off the bat. It has the planning stage, double crosses, more planning, really bad consequences, and the like. Also "Munich" is a good example of real life missions. Its not as detailed about the plans, but still a good movie. And check out some table top role playing game forums. Every role player has stories of horribly bad plans, well thought out plans that failed because they forgot a flashlight, and beautiful plans that had everyone grinning like idiots as they marched to glory.

Good luck