what do you do about "bad meditation" days?

[anagogy]

so what do you do when your daily meditation does NOT go right? Is it ever so bad for you that you could barely sit through 20 minutes?

I find forcing it to be counter productive. Just do something else. Simply intend that you will try again tomorrow and that you will be really focused and in the mood to meditate. You would be surprised how effective a firm instruction to yourself can be. It gives your subconscious something to fixate on.

Alternatively, do some exercise before you meditate. Even just 10 push-ups makes meditating at least 10 times easier. Usually when you can't sit still its because your body doesn't feel like sitting still, so you need to expend some physical energy to properly utilize your mental energy. It will really help you get in the flow of things.

edit: I also wanted to add this goes doubly true for magickal work (i.e. manifestation). Don't attempt to create new "belief bridges" when in a resistant negative state. It won't go well. It is a *lot* of work for little gain, wait till you are in a less resistant state and you will make twice as much progress. Quality over quantity.