Create Reference "Cage"

You will find that you will create geometry separate from your design primarily as a reference for position, compare, or separate objects. A good "reference cage" includes both geometry to reference the "dummy object" and workplane and the workplane of the object being modeled. (Here's a hint -- a reference cage for a cavity emphasizes the points and curves inset from boundary between cavity and dummy while a reference cage for an outer surface emphasizes the points and curves offset from the boundary of the object.) 

Triangulate Placement Using Orthographic Projections

By connecting perpendicular lines across between two orthographic projections and then extending a perpendicular line from a third projection to where crosses the first line, it is possible to locate reference points from your design precisely in 3D dimensional space. 

Use Many Layers (Duplicating, Locking, Hiding)

Get to know the layers in your application and develop a practice of duplicating all of the elements that you will use for joins, surfacing, extruding, boolean operations, or positioning. 

"Reference Layers" are extra layers that contain only the points, curves, and surfaces that you use to produce solid parts by combining, lofting, slicing, and boolean operations are of crucial value later in the design process as a reference for how you created complex shapes. Create separate reference layers and copy and paste the elements you use as tools before running operations on them so that you can retrieve them later if something goes wrong and you need to roll back the clock.

Construct Complicated Shapes From Primitive Solids 

Building organic shapes from simple primitives, curves, lofting, and rails keeps the complexity of the model comparatively low, allowing for better control over adjustments. The best place to start is at the sketching phase, to locate the primitive solids that get close to solving most of the construction issues. These elements can be quickly constructed in the modeling phase, getting you most of the way to the intended version with fewer control points required.

Many Solid CAD packages have a series of tools to fillet, chamfer, and bevel edges and corners, which can go far to tuning up the results of these first quick constructions. 

Build Details Using Points, Curves, Surfaces

Almost all CAD tools offer the ability to construct solids based on points/vertexes, lines/curves, planes/surfaces. Where they differ tends to be in the manner of control and adjustment. 

Whichever tool you select, consider first how you can move from each of these construction tools towards solids and back towards more primitive elements to allow for precise editing throughout the entire process. 

Grouping vs Joining

While the precise name for these two features differs from package to package, "grouping" and "joining" are the most common names for these related but dissimilar way to bring together elements. 

• Grouping means to collect an assortment of selected items so that they can be manipulated as one entity, handy for a host of transforms such as scale, rotate, move, copy, and hide. Groups can also sometimes be assigned to special colors, beyond the color associated with the layer.
  
• Joining means to process the sub-entities into a new conglomerate entity: example, lots of little lines becoming a closed outline. While this kind of operation can be essential to perform transforms such as various "Extrude" commands or "Create Surface from Curves," it might not be easy to revert to the many elements you used to created the joined element. When in doubt, group all of the elements in question, duplicate them a reference layer, and then perform the join command on the original set of the element.  

Boolean Operations - As Late As Possible

One of the best way to combine your primitives into more elaborate shapes is to apply boolean addition/union and subtraction/difference to clusters objects. However, as you perform boolean operation after boolean operation, you forever alter the system of control points that can be accessed to make changes to the form. 

To protect against making mistakes that force you to rebuild a model again from scratch, it is important to duplicate and preserve the pre-boolean shapes in other layers. Typically, I will continue to add solids into a group until I am certain I am ready to union/difference them, at which point I'll duplicate the set to store them for later, and perform the boolean operations systematically in one go. It is helpful to check to make sure the resulting model is manifold after performing lots of number crunching -- Rhino OSX has a Volume check under the analysis tool that will tell you if your volume is water-tight as well as its size. Blender has a check for manifold option as well.

Printing Test Parts / Insertion Test

As early as possible, it is important to start testing your design in the real world. That is, in fact, the best reason to have a desktop 3D printer!

I didn't like the outer contour of this version of the watch, but I closed it up quickly so I could print a model to test for fit when inserting the electronics. I discovered immediately that my factor for tolerances was too low -- too tight a fit -- and I made changes to both the cavity side and outer shell side to match the new measurements.