Forms - Each form should have one central theme. Add forms rather than clutter up the mainform. Controls - Buttons should follow a consistent pattern throughout all forms. Buttons in a vertical stack imply a sequence of events that must be followed from top to bottom. Buttons in a horizontal stack imply a sequence of events that must be followed from left to right.
Consistency is the key here, consistent size, location and logical grouping.
This means adding access keys, and where possible shortcut keys to menues, labels, etc. The tab order should be set logically and include only input controls. It makes no sense to be able to tab to a read only control. Perform a quick research study on all the GUI applications you use and determine if there is any agreement between those applications pertaining to the area of GUI design in question. If there is a general agreement, then use it, there is always safety in numbers.