You've just had your shower retiled, and you have to put in a soap holder. But you don't want to drill a hole in your expensive new tiles. You could get one of those suction-cup soap holders, but: meh. Instead, use the power of magnets! There's no penetration through your new tiles, it's easy to keep clean, and there's no mess if you change your mind.
Step 1: Prepare the Tile
So: jump in your time machine and go back to before the tiles were attached to the wall. Dremel out a couple of holes, side by side, for magnets. I used C-shaped magnets out of some old hard disks (see separate instructable for removal tips), but you could equally use new magnets bought over the internet.
You'll want a diamond bit for the Dremel. Run the Dremel at its highest speed and use a light pressure. Maybe use water to keep the job cool. When dry-fitting the magnets, definitely ensure that the job is reasonably cool first -- heat denatures neodymium magnets. One of mine is a bit weaker than the other, and I suspect that I got it a bit too hot during dry-fitting.
The big 300 x 600mm tiles I was using were thick enough that I could get the magnets flush. If yours aren't, you could just make a dip in the render where the magnets are going to go -- that will let you make a shallower hole in the tiles, or maybe no hole at all.
You don't need much adhesive to hold the magnets in the tile -- it only has to be strong enough to hold the magnets in during installation. Thin double-sided tape or just a piece of duct tape over the magnet would probably be fine. I used a little bit of five-minute two-part epoxy. (Hot glue is probably not suitable -- the thermal mass of the tile is very large, and will cause the glue to cool rapidly, probably giving you bad adhesion. Also, hot glue is hot.)
Step 2: Make the Soap Holder
Place the magnets on the wall and wiggle them around until they are in the place they most "want" to be. This shows you how the magnets should be placed on your soap holder.
For my first attempt, I bought the lightest soap holder I could find. (This was probably overkill.) I reworked the back of it to give me a flat space, and attached the magnets using Sugru. I put tape over the back of the magnets, sticky side out, and wrapped the tape around the soap holder to keep the soap holder attached to the magnets while the adhesive set. This ensures that the magnets are in the right position, and are both flush to the wall -- doing it this way, it's not possible to set one magnet in crooked.
Note the "prong" at the bottom centre. The easiest way for the soap holder to fall off the wall is for the bottom of the soap holder to act as a pivot point, and for the weight of the holder plus soap to shear the magnets a fraction of a millimetre away from the wall. This weakens the magnetic force, and you're in a very quick feedback loop. Moving the pivot point further down the wall proportionately increases the force needed to shear around that pivot point.
Step 3: Make the Soap Holder Again
I wasn't happy with that first effort -- it worked, but didn't look as good as I wanted. Eventually I figured out that I had to look at the next shelf across at the hardware store, and get a soap holder designed to sit on a vanity, rather than one that came with mounting hardware which I had to cut off. So I got a bamboo one.
I chiselled out a piece of the bottom to make it lighter. Again, this was likely overkill, but it did provide me with a piece of wood to make the prong.
I chiselled holes for the magnets, flush to the surface. I used a little bit of five-minute two-part epoxy to hold the magnets in, then coated the whole thing with Estapol, a polyurethane satin finish. This was a mistake -- I wanted to fill the cracks around the magnets flush to give no toe-hold for mould to establish itself in, but Estapol shrinks a lot when setting, so I had to do about ten coats around the magnets. In hindsight, I should have filled with the two-part epoxy or wood filler or whatever, and then finished with the Estapol.
Step 4: Conclusion
I'm happy with how it turned out.
If I did it again, I'd put more magnets behind tiles, so I had a choice of heights for the soap holder (my girlfriend is shorter than me) and maybe a spot on the other wall.