When working with large panels, it can often be difficult to ensure the entire surface is sanded evenly. A little extra time in one corner of the board with a power sander can leave hills and valleys. These defects may stand out when finished, especially when finished with a glossy finish.
An easy way to make sure the surface is sanded smoothly and evenly is to mark the entire board with a pencil. The marks will disappear as you sand, revealing areas that still need attention.
This tip can also be used to level a board. Use a straight edge such as a framing square to identify the "high areas" on the board and mark them with a pencil. Sand through the marks and repeat the process until the board is level.