I use Blender to map all my models and although I am better at it now, it did, like everything in Blender take a while to get used to it.
 A tip with seams is to check that they are continuous by selecting a face and then using CTRL+L to check them (Only faces up to the seam will be highlighted). Any break in the seam with cause the whole mesh to become selected instead of the island.
Alternatively, set up vertice groups while modeling for easy selection.