Stick a 4 foot level on that building board and make sure it's positively dead
FLAT
A FLAT board is your BEST tool in building kits. Flat boards make straight wings and straight fuselages. Warped boards make planes that fly crooked.
Use LONG sanding blocks. A small hand held palm sander does nothing but sand waves and warps in your model. My favorite sanding block is a piece of MFB countertop 3/4" thick and 4"x11" Wrap a piece of sandpaper around it and hold it on with your hand. This long sanding block makes nice finishes on sheeted wings and tails.
Also the Great Planes easy touch sanders work great and come in 3 or 4 different sizes:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXK317