The people who tend to question my tricks will usually do it regardless of how it's presented.
@Antonio Diavolo, do you typically perform for the same group of people and it typically is the same individuals who question your tricks? I'm asking because the more you perform for the same people, especially people who know you well, the sort of "fool us" vs. "guess the method" contest can become common.
What I find funny is that as magicians, we tend to do the exact same thing we don't like our spectators to do. We watch with a goal of figuring it out.
If they are speculating on method, it is because they think knowing the method is more interesting than seeing the magician perform it.
Either this is because the performance is set up in such a way that they feel they are meant to try to figure out the method (ie: the performer has challenged them in some way to do so), or because the performance is boring and figuring out the method is a way to entertain themselves while the magician faffs about in front of them.
In either case, the performer can stop it from happening by changing how they present the performance. Therein lies the difficulty for most magicians.
There are two ways of doing this and they are best used together.
The first way is the design of an effect. The effect needs to be designed to subtlely disprove all possible methods, including the method used. I say subtlely because if you obviously rule out a method (all the cards are different, this is a normal deck, etc.) that focuses the audience on the fact that there is a method -- making them wonder what the method is. In many ways, a magician's attempt to be "fair" (please, never ask a spectator "was that fair?" unless your next line is "of course it isn't, I'm a magician, I cheat"), can backfire because it makes spectators focus on what your are doing and think about how you could be being unfair.
The second way is to present the effect in an entertaining way so that they willingly suspend disbelief -- that is, they don't care how you did it because it was so much fun to watch your performance. Structurally, I've always felt that you should have the audience expect the final reveal of an effect a moment before you get there, to WANT the magician to be able to meet their expectations and to be convinced that the magician cannot do what they want him or her to do. That's magic.