First up, insulation!  I was able to get a great deal on factory seconds of 3" thick pieces of foil faced polyisocyanurate rigid insulation, which provides an R-value of 6 per in.  The previous owner had insulated the floor with fiberglass batt's (the pink stuff) which isn't the best choice for tiny houses.  The vibration when moving the house can cause the fiberglass insulation to settle/move, resulting in reduced R values.  That, and he had installed it before putting the roof on, so the majority of the insulation had gotten wet and was beginning to mold in places.  After removing the old insulation, and replacing some of the subfloor material that had started to rot, the floor was installed.  Next came the walls, and then ceiling.  Any gaps were filled in with Great Stuff spray foam.  It took a long time cutting all the insulation to size and filling in all the gaps.  The result was a huge mess, but a very tightly insulated house.