I will preface my entry by stating very clearly that this was my first attempt at tiling. Therefore take all my tiling wisdom with a grain of salt. To mangle an overused quote:
Since my tiles were massive 13" tiles, I installed a horizontal support using a straight piece of scrap and started working upwards. The cutout for the soap dish was easy enough, but when I reached the level of my glass tiles, I started to run into problems. First, the glass block seemed to sink in further than the tiles and when I continued to lay tiles above them, the upper 13" tiles would slide downwards and compact all the glass tiles! After a while of panicked attempts to repair the situation, I could see that I was getting myself further into a hole, so I decided to pin the upper 13" tiles in place by minimally inserting a concrete screw below them, then peeling off the glass tiles, and scraping clean the mortar that was in the gap. At this point, I called it a day and proceeded to cleaning up the glass tiles before the mortar could cure any more (it was starting to get sandy).
When I returned to the project the next day, I discovered my second mistake: I had forgotten to cutout a space for the corner tray at the front of the shower! I had only discovered this error when I returned to the wall on the second day. My compromise was to install it lower, at the level of my support bar which was now removed. My wife also told me afterwards that she had wanted two corner shelves, one at the front and one at the back, however I had not picked up two shelves... Hindsight is always 20/20.