This is a LONG post, if you want to skip ahead to the best stuff in it then scroll down to Without a Cover #2.
First, let me echo others in saying that the Gertner transformation is beautiful and practical and you need ought to at least be aware of it if your purpose is to provide your audience with the very best effect.
A few ideas, first, involving a cover -- not only because they can be more practical -- but because there are some interesting visual effects that can be achieved.
Unlike Gertner's handling -- which is the result of careful thought, experimentation, and which has been honed by actual experience in front of live audiences; these are very raw ideas. If you like anything here, there is a lot of work to be done in experimentation and polishing.
With a cover #1:
A small hourglass is on the table. Performer waves a cloth (or crown royal bag -- another excellent suggestion on Curtis' part...though I am not sure that such a bag will provide adequate cover in this method) in front of the glass and it transforms into a ten incher.
by adding a small "tab" to the top of the 18"er you can use Bob Read's exceptional bottle production to steal the large glass. The bag/cloth is held as per Read, but rather then being clipped by the thumb it is clipped between the middle and fourth finger -- freeing the thumb to clip the tab during the steal (from the front it appears exactly as Read's handling) The extension of the arm (which Read motivates by removing the nose) is motivated by setting the small glass on the table.
The small glass is set immediately in front of a black art well. As the cloth is lowered over the small glass, the small glass is nudged into the well and the large glass is left on the table.
The best technique on this cannot be worked out just in the mind. You need to set up a video camera and experiment with the props in hand. Keep in mind that depth of field collapses onstage -- so, with practice, it shouldn't be hard to make it look like the large glass occupies the same place that the small glass did. Also keep in mind that, for the change to look visual, the motion of the hand must be continous. It doesn't need to be fast, but there can't be any jerky stops and starts.
With a cover #2, a vague notion:
A small hourglass is in the center of the table. Performer allows a cloth to fall over it, as the cloth descends it catches the shape of the small hourglass...he lifts the cloth and drops it once more...and then again -- but on this third time the cloth forms the shape of a much larger hourglass
I have no specific technique on this -- but I like the idea. The way to find this technique is to perform the actions in front of a camera, ignoring any thought of the secret. Then study the tape, looking for the right moment -- in the course of the natural actions -- to steal and set the large glass. Keep in mind that you are one ahead here -- the moment the large form is seen under the cloth the effect is perceived to be over. Also, had I the mechanical wherewithal, I would not be averse to using an automaton table...
With a cover #3:
Adapt the methods of Ross Bertram ("My Favorite Drink," p. 115 The Magic and Methods of Ross Bertram or Lennart Green (on the first volume of the A-1 video series) for transforming an empty glass into a full glass while covering or polishing it.
Without a cover #1:
There are, again, a lot of nasty technical issues that need to be worked out through actual experimentation. The basic concept is this:
A small glass is on the outstretched palm of the performers right hand at about waist level. Both hands shoot up towards chest level, as they move the audience sees the glass visibly grow. When the hands stop a large glass is between the palms, at chest level, and is extended away from the body in an open position
The set is a part of the deception here. There is a chair or similar object immediately behind the performer. If a chair, the back is to the audience (perhaps the performer's case is on the chair). The large glass is sitting on the chair or is in the case (provided it is a doctor's bag or similar design that stays open). The height of the glass and chair is such that the top of the glass is at the right position to be lightly grasped by the left fingers when they are hanging naturally at the sides.
There is a holdout with a line down the right sleeve and and an attach to connect to the small hourglass.
You hook up naturally (this needs to be thoughtfully worked out in front of a camera). The small hourglass is attached to the line and in the palm. The left hand straigtens your lapel and then falls to waist level.
The left fingertips (lightly!) grasp the circumference of the large glass resting on the table.
Both hands shoot upwards and forwards together. The small glass goes up the sleeve whilst the large one is placed between the hands.
End the motion in a very open position, with the hands far away from the body (this should naturally pull the sleeves back, suggeting that they cannot be the place to which the small glass went). Cf. Gary Kurtz' Leading With Your Head.
One of the major weaknesses of this method is the fact that the left hand has to be concealed behind the body. No matter how naturally your position, it's gonna look a little bad. Also, because of the geometry of it, the change isn't going to happen with you full front to the audience, so it isn't going to be as visual as this next item. On the other hand, the steal from the set-piece allows for a much larger glass than you may be able to conceal on the body.
These considerations lead to my favorite method:
Without a cover #2:
Here we bring the action to a smaller stage, between the stomach and the chest, and we are full front to the audience so that we get the biggest visual punch possible.
On the right side of the body is a pendulum holdout -- designed as per Tommy Wonder (for this effect we really do need the small glass to shoot out of sight as quickly as possible and we need it to be as far from the edge of the coat as possible -- Tommy's brilliant design accomplishes both ends).
The large bottle lays over the left buttock. It is attached to the belt with the hook-up used by Barrie Richardson for the bowl production kicker in "Ovation Position" (Theatre of The Mind, p.5). There is a small ring attached to the belt, into which fits a hook (crafted from a clotheshanger) that is attached to the circumference of the larger glass.
The small glass is on the table. There is a small piece of white satin tucked into the back of the pants on the left side, above the large glass.
The hands come up to adjust the lapels, as they do, the right hand steals the attachment on the end of the pendulum holdout into the base of the right fingers. The left hand reaches and takes the small glass from the table -- the right hand staying the lapel. The left hand brings the glass into the right where it is attached to the holdout.
The performer looks closely at the glass and seems to see something that bothers him on the surface -- a speck of dust.
The left hand reaches back and takes the small piece of satin (it appears to the audience that this has been removed from the left hip pocket, a pocket which has been rendered inaccessible by the load)). The performer lightly polishes the glass (while saying something interesting!).
The cloth is held between the first finger and thumb as it is put away (pushed back into the pants)...as soon as the front edge of the cloth is sandwhiched between pants and body the thumb is freed. While the first finger pushes the material in deeper; the middle and ring fingers -- along with the thumb and pinkie -- grasp the circumference of the large glass and lift it an inch or so -- disengaging the hook from the ring.
The pinkie and ring finger kick back so that the glass goes in a horizontal-ish position -- nearly parallel to the waist. The hand makes its way around the back and rests on the left hip.
To the audience it appears that you put the cloth away and are now resting the left hand on the hip. Your first and second fingers are visible. There is going to be some tension because weak fingers are holding a reasonably heavy object in an awkward position -- to circumvent this you need to completely relax the muscle that runs between the neck and shoulder.
Your weight must be back on the rear of your left foot for this position to look natural.
When you wish to affect the change:
Both hands move forward sharply and to chest height sharply. The ph rapidly gets rid of the small glass as the left hand brings the large glass into view. Finish in the open position described in "Without a cover #1."
This is a strong and practical method -- there are details that remain to be worked out..but the meat of a beautiful transformation is there
of course, Gertner's version is exceptional and has actually been performed -- why not start there?