Sleepy's Guide to Dealing with In-Laws (the Abridged Version)

This is a quick, condensed (yet fairly comprehensive) list of advice of do's and don'ts when it comes to in-laws.

1. We've all heard the saying, "take whatever (name) says with a grain of salt."  Well with in-laws, the rule is similar but not quite the same:  Take (enter in-laws name) with a shot of whiskey.  Or a xanax.  Or a valium.  Or a beer.  You get the point.  Chances are there will be times that your in-laws will say something demeaning, out of line, or horrid to you.  Take it, not with a grain of salt (because really, what is salt going to help?) but with your favorite relaxant or tranquilizer.
2. Accept that you are not good enough for the person you married.  I know that you are saying, "Sure I am, my in-laws and I get along fine.  They like me, I'm a doctor/lawyer/other successful person."  I am here to tell you, they do NOT like you.  All you have to do to bring out the venom is cross them, disagree with them, or move their "baby" away.  The true colors will come out.  I promise.
3. The only joy or happiness you can give them is a grandchild.  After the child is here, you are now rendered useless again.  You are no longer capable of making rational or logical decisions for your child or your spouse anymore.  They will know what is best because you are stupid.  Everything you do with their grandchild is dangerous in their minds because you are incompetent.  They do believe that they can do the same exact things you do without it hurting the child, but sense you are an incapable idiot you cannot. 
4. They are not logical creatures.  They cannot be reasoned with because they know no rationale.  Any effort you put forth to explain yourself about any topic is all in vain, a big waste of time.  All that you can really do to apease them is to shake your head and smile.  Sure, you compromise your integrity but at least you get to walk away.
5. Control, or at least percieved control, is important to the in-law.  They like to know that they can still have some influence over their child, especially if it is influencing them against you.  So, tell your spouse to implement the same technique as listed above (nod head & smile technique) if they want to get out of it quickly.  If they want to go the route of standing up against the parent, then they need to be prepared for the bombardment of wrongness that will be doused upon them.  The in-law, when dealing with their offspring, is not afraid to use guilt or shame to attempt to influence.  As a matter of fact, that leads to number 6.
6. Guilt and shame are the favorite weapons of the in-law.  Sure, they use it primarily to dominate their own offspring but they are not afraid to break it out on you as well.  They will guilt you about anything you may have done wrong, but the main weapon is shame.  The shame for who you are, what you have done to their precious offspring, they look at you with those eyes knowing what happens behind closed doors and locked bedrooms.  Yes, the shame.  The shame is like a loaded gun with them.

While this list is not a full and completely comprehensive list, it can still help you if you have in-laws.  Remember, you might think right now that your relationship with them is fine, you might think that this list is of no help to you, or you might think that I am exagerrating, but just wait.  There may come a day that all of this comes out and hits you like a ton of bricks.  They may have you lulled into believing that everything is okay because you have been nothing but compliant with them so far, but cross them, I dare you.