If I'm struggling with a particular card, I'll usually go look it up on a good comparative tarot website to see if I'm missing something. I'll take another look at the cards around it, and also consider the cards that come before and after it in its suit. If its a 'negative' position, I'll consider the card's usual reversed meaning.
Clarifiers almost always just confuse. There are only 78 cards in the deck: that card is meant to be there, and adding another card just dilutes things.
If I really do need more information, it depends. If everything is seeming a bit obvious, I might take a shadow card from the bottom of the deck.
Usually I fan my cards and select them instinctively.
Sometimes I'll write down the spread or take a photo of it (love my digital camera!) and re-shuffle and re-lay a the spread to see what cards come up, to see if any are repeated. On occasion I've also laid additional cards from a different deck. But generally, I find that the best thing is to read what you've got.
