1. For the most part, don't bother pouring outside or signal layers on a multilayer board. Can be helpful in demanding cases, but isn't worth the trouble for the most part.
2. In 2-layer designs, pour the same net (GND) both sides, and stitch with vias. This connects peninsulas, crossings and islands, and provides a ground nearly as good (in terms of impedance and average distance from trace to ground) as a multilayer board.
3. Any islands remaining, should simply be too small to bother pouring or connecting (set a minimum island area threshold).
Tim