One routine that I use, that gets 'em everytime, involves my special card - the three and a half of clubs.
Force the 7 of clubs onto a spec - don't let them see it and place it face down on the table. Next, force the three and a half of clubs onto another (or the same) spec and place this face down on the table too.
With both cards face down on the table, point to the three and a half of clubs and patter on to the effect of, "okay, you've both chosen 2 random cards. What would be the odds of this card being exactly half of this card" (now point to the 7 of clubs) ?? You clarify the situation by explaining that there is no play on words, so if, for example, the first card is the 4 of hearts, then the other needs to be the 8 of hearts.
You now let the spec pick up and look at the 7 of clubs and ask them again, what they reckon the odds are - I usually ask them would they be willing to bet on the outcome ?? I do, however, stress that I won't take their money should they lose - but would they bet if I was willing to take their money.
I tell ya - I could have made millions (okay, slight exaggeration) if I'd wanted to. It's more than enough though, watching their faces when they turn over the 7 of clubs and then you reveal the other card to be the three and a half of clubs ...... priceless

.