I think this is due to the much underlooked magical skill, "selecting your audience and time!" The first time I performed this, I did it for my "acid test" - my 15 year old brother, about 3 feet away, in an intensely bright bathroom with mirrors in it! Suffice to say, he wasn't fooled. Luckily, he's my "examiner" and "inside spec!"
If you perform almost *any* close-up magic for a hard-thinking crew who are in the mood to play devil's advocate, to not respect your right to magical secrets and to happily spoil a good trick by being smart-arses rather than keep the spirit of illusion alive, you are asking for it!
"A prophet is not recognised in his own country", and indeed, the worst spectators for your magic are your close friends and relatives, who know that you leave the toilet seat up, fail to wash your dishes and forget your keys along with the best of them! Random specs: fine. For all they know, you are a man (or woman) of mystery with fantastic abilities to do impossible things. Your friends? To them, you're just *you*.
Ok, so you could say "practice more, it should be totally natural, there should be no hints" etc. but come on - if you were simply to repeat the actions of a magician pulling "Sinful" you would probably click onto the initial move straight off, and probably a couple of other key bits as well. That's not accidental exposure on your part - that's the wrong audience!
Ideally, when you do a trick, consider the following things:
1) An agenda. If there is a "schedule", even if it be "one of us needs to get off the bus soon" or "I've got more tricks to show you", you are putting a natural time limit on the time they have to mull it over. Every good magician knows the line, "why don't I show you something else first?" when asked "do it again!" - this is the simplest way to stop them sitting there, trying to work out how you did it - or even having a go themselves, or asking to check your pockets, or whatever! If you do a trick for a group of intelligent, sober people, and you only intend to do one trick, and you do it near the beginning of however much time you're spending with them, you're asking for trouble!
2) The type of people you're performing to. As a scientific-type, a lot of the people I see day to day are so rigorous/an*lly retentive that they do exactly what your friends did - they think, "well, obviously such-and-such couldn't happen, so it was obviously this..." - the old Sherlock Holmes principle! Preferably, your audience will include a smattering of people who are a)young, b)gullible or c)inebriated enough to blow the illusion you just did way out of preportion. David Blaine doesn't go to MENSA and do magic, he chooses people who he knows are going to go "WHAAAAAAAA?!?!?! How the f***, man?!!!! Impossible! I mean, WOW!" because once the effect has been pulled, they are doing half of the hard work for him in convincing everyone present it was a total miracle. And they will ask the "right" questions, the questions we love hearing, like "can I look at the can/coin?" as they try to catch you out at a certain point when actually you are so far ahead of them it hurts
3) Finally - the beauty of a "routine", rather than one off tricks, which doesn't come across in stand-alone DVDs like "Sinful". The aim is to leave an impression on your specs that you did something incredible - this
will include a particular effect, but not necessarily the details. Eg. my friend saw a magician in a club do the ID. She was amazed - and when she describes it to me now, she tells me that "all I did was think of the card - I didn't even tell him, he read my mind" etc. Her memory is of an effect, yes - but the overall effect has overpowered the fact that actually, she will have told him which card she thought of before he produced it! His general skill/ability was what came across. Likewise, when you do "Sinful", the aim is to leave people saying "he slammed a signed coin into the can and it went inside, so that when he opened it..." etc, not "he whacked his hand into the bottom, then showed it to be empty, then passed the can from one hand to the other..." - see the subtle difference?
Anyway, this is way too long. But to summarise - do still keep practicing, it never hurts! And try to think what's coming next and how it will be received, before you do a trick like coin thru can.
All the best
