When you grout, make sure that you have the right tools for the job - a sponge with a mesh backing and a small pick. Start by cleaning the tiles with water and a mild detergent.  Use the scrubbing side of a sponge like the one pictured below and be sure to clean any mortar left behind in the grout lines with the pick.

When you are finishing a shower niche, lay the final tiles in this way:

• top left side tile
  
• top niche tile
  
• top right side tile
  

Doing it this way ensures that the top tile will not fall before you get both side tiles in place.

When installing large tiles like 12x24 on a ceiling, ensure that your thinset mortar is mixed to a thick consistency.  Otherwise, you will need a hard hat as you can expect the tiles to fall on your head!  In addition, make sure you set the tile with some serious upward pressure for a good 20 to 30 seconds.  Visually, plan out your tile placement so that the joints on the ceiling tiles are offset from the joints on the wall.