As a few people have said, always have a backup normal deck in your pocket. I am never seen with a gaff deck and without a normal, identical one. I am not that big of a fan of the Svengali deck just because it is available in the "Marvin's Magic" series making it very well known, maybe it is just my paranoia but I do not like to perform too many things which are too easily available to the lay public. I do however have my ID with my almost 24/7, I keep it in my back right pocket and an identical un-gaffed deck in my back left pocket for a quick switch. I perform the ID replace it into the box and replace it into my pocket while the people are talking, I do this quickly, but VERY casually calling on no attention to myself (it is OK if they see you doing this, but if you do not pay attention to the move, they won't either), then (if and ONLY if) they ask to inspect the deck I pull out the normal deck which is in my back left pocket (even though one or two smart spectators might have caught a glimpse of me putting the deck into my pockets, they notice that I put it into my back pocket and not which back pocket I placed it in). If they do not ask to see it, I pull out the normal deck anyway, but instead of letting them inspect it I move onto another trick.
Also, like a few people have already stated:
When I first started out I always had people asking to inspect my deck (when I first started out I used a stripper deck a bit) and I could not give it away. I had no clue how to handle these people and would just say "No, this deck is special to me." and I would put it in my pocket and either walk away or pull out a COMPLETELY different deck...

.
Now, barely anyone asks to inspect my decks (or anything for that matter) unless I do a very long performance. In a long performance you just get the "Well, this guy has done 10 tricks that there is no <excuse my language> way he could be doing this, his deck must be tricked." Normally I even get this to happen when I am already using a normal deck so no deck switching involved. When I am using a trick deck however I either say "Sure, just let me show you this last very cool trick first." and during this I quickly, but once again casually (making sure they SEE you placing it in the box, place the deck in the box (slightly secretively, in my back pocket, say a joke for misdirection, pull out the normal deck at this point, and pretend like nothing has happened. I then open up the box, trying to make everything look like it did when I showed them the gaffed deck, and then hand out the normal one I now have in my hands to the spectator for examination. I then move onto a trick involving a regular deck of cards.
Also, on a slightly off-topic note although it is still pertaining to cards:
Do you find yourself having excessive amounts of cards on you at ALL times? With me on a normal day (some times when I am having a "I don't want to do magic today type of day I will only have one deck but otherwise...) I have 3-5 decks in my pockets and more in my backpack/coat/suitcase/car and sometimes have 3 or so dupes per type of deck I have with me. Today I found myself with...
1 Black Tiger ID in my back right pocket.
1 Black Tiger Normal deck in my back left pocket.
1 Black Tiger Stripper deck in my front left pocket (for the smaller kids for which you need some "instant" tricks that seem impossible).
1 Black Tiger Svengali deck (for the same reason as above).
1 Ghost deck in my sweater pocket.
1 Bicycle Master's Edition deck in my backpack.
1 PK Ring on my finger
And finding myself reading PRISM in between classes and in the middle of class using up every second of my spare time.
WOW.
And on a side note to that comment, PRISM is highly recommended and as soon as I finish reading it I will post a very lengthy review. From what I've read so far though, the first trick (at least for me because the concepts in it where new [even though slightly basic], combined more than one concept to make the effect even more killer) is worth the price of the book.
Anyway I will shut my mouth now and quit rambling on and on. Sorry for making this so long.
Hope all of this helped above my "Off topic",
~David
P.S. Somehow the words "I also believe" ended up under everything that I wrote so if any part of this post is missing those words, please let me know so I can edit it!