To grout internal corners, it's a good idea to lay the tiles on eah side of the corner in pairs.  This makes it easy to obtain a uniform horizontal joint.  Using sealant along the corner will protect it from water, and accomodate for movement between the tiles.

Bathrooms and kitchens are usually damp, and conducive environments for mould to grow.  The mould manifests as dark stains that are virtually impossible to remove.  To avoid this problem, it is advisable to apply a fungicidial based solution before tiles are grouted.

Don't let grout harden before cleaning the grouted tiles.  If the grout does start to harden, a scouring pad will often do the trick, as long as the grout has not been left to harden for too long.  Also mix a small quantity of grout at a time, to prevent premature hardening.