Assembly

Have all materials on hand - wood, glue, clamps, clamping blocks, straightedge, etc.
Label parts before assembly (top, bottom, side, ...).
Sand as much as possible before assembly
Dry fit joints before gluing.
Use spacer sticks to maintain the appropriate spacing and layout of pieces.
Do not overtighten clamps when gluing pieces together.
Use wood scrap blocks to evenly distribute clamping pressure.
Cover metal clamps with wax paper to prevent stains at clamp/glue interfaces.
Glue up complex projects in sub-assemblies.
Check for square on boxes, cabinets, shelves, etc. by measuring the diagonals.