Placement of your gimmick depends greatly on how and where you use it as well as the style of device you employ; concealing a scarab (Boon) would be far different than how you would a TT Writer.
The Boon styled writer must be set somewhere that keeps the adhesive from getting dirty and thus, not sticking to you when needed. There are a number of "Hold-outs" for them but you need to look around in that such concepts are discussed in this or that book and/or videos. I believe it is the John Rigg's "Gods with Feet of Clay" series, disc 4 that has some in-depth discussion on this.
I tend to use either a TT style or Banjo Pick styled device when I use them and other than my pockets (and I tend to have one in at least three different pockets) I have a special way of concealing one or two either in my cane (for when I'm walking about) or in the arm-rests of my wheelchair.

I knew a Carney long ago, that would place a bit of "Nose Putty" behind his ear and that's where he would stick his gimmick. This way, when he took the pencil from his ear and put it back, he could almost instantly steal out the Swami or ditch it in a very natural way. I don't think he ever got found out.