Big Dan wrote:though we all know that this is rarely the case
Erm...?!
95% of the tricks I perform are totally impromptu. In fact, the only things I do that aren't are NFW and the BCS stack. You should really move onto some truly impromptu card magic.
For example, although we may consider it simple, try this one.
Have a card selected. Control it to the top. Snap your fingers and show the top card wrong with a DL. Give them the top card and tell them to snap - tada!
Also, for moments when you're caught withough anything, borrow a 2p coin. You can do a lot with that! (Retention vanish and produce from their ear with a Downs Palm or Goshman Pinch is always a good one!).
Just some tips,
Kati