The Secret Guide
AKA - How to avoid pissing off your grrlfriend/wife/ lover/mistress/whatever by just not sticking your foot in your mouth, thereby preventing hers from coming into contact with any part of your body. Hope it helps.
::)
Chapter One - Speech (or perhaps lack of)

    One common mistake is the inability to just keep our opinions to ourselves, now some of you will start out here with at least enough brains to know that if your SO (Signifigant Other) asks a stupid question like:
      SQ ) "Does this dress make me look fat?"
   You will not answer with:
      SA ) "No honey, the dress is fine, your ass makes you look fat."
      SA ) "Well, maybe a little."
   These answers are equally bad and will cost you - even said in jest. What you need is a perfect answer. AKA - A lie or redirection. If the red dress your SO is wearing makes her look like she is promoting a barn sale... Suggest another dress that you know looks good/great/not embarrassing in public OR just lie, say no and take your punishment.
    So that isn't enough of an example to save you in every situation? That is why this site is here. Appropriate answers to common and trick questions will be answered here. All this will be expanded to the maximum allowed. (Much like a grrlfriends ass when you marry them, they are no longer your g/f, they are now their mother)
So you've read this and you think I'm a really insensitive bastard? Read the FAQ!
Problem: SO doing something (watching TV, staring into space, or some other activity/inactivity) with sour/sad/unhappy look on face.
InitialAction: You've asked what's wrong and gotten one of these replys:
a) Nothing
b) Shakes head (as in no/nothing)
c) Other inane reply such as "I'm just tired" which really means "There's something wrong, you don't know what it is and I don't want to talk about it because it's silly/not important/probably not something that you could do anything about even if I told you you stupid jerk."
d) Blatantly ignores you when you say things (anything).

Actions to avoid:
a) Dismiss it as PMS.
b) Dismiss it as not important (If it was important she'd want to talk about it right?)
c) Posting the problem/solution on a web site...

Action to take: Try to think of any earlier conversations that may have taken place about things she may have been uncomfortable with. Then bring up what you think it might be. You can also try bringing up a lot of other things you know it's not just to try to lighten her mood (such as - "Your (in this mood) b-e-c-a-u-s-e McDonalds has served over 1,000,000,000?") sounds silly but it can work. Discuss what her fears or worries are and put each one to rest with appropriate information (or outright lies if you have to - try to avoid that if possible though because the consequences could be a new G/F) and then do some follow-ups until the event that is causing the issue is past or avirted to keep her mood up. Afterward make sure that you let her know how much you missed her (If she didn't go) or make sure that you do the old "now that wasn't so bad" followup if she did go... Which means I should explain the event causing anxiety - First Fathers Day w/new G/F nervous about kids/ex/family/extended family and how she fits in with everything...
Legend 
PROBLEM
BAD ANSWER
GOOD ANSWER
Good and bad ways to respond to your SO doing something stupid:
First - The bad ways...
a) Tell her how stupid it was.
b) Bitch and complain about how stupid it was.
c) Bring it up all the time, reminding her daily of how stupid it was.
And now for the good ways (which may prevent a fight)...
a) Let her know that you care about her and would like her to avoid (the stupid thing in question) because it's better to be SAFE than SORRY. <-- That can be put better, but, I'm still pissed.