A Few General Modeling Tips

• Print a test object from your design software. As a first stage to design, create and export a small file with some easy to measure parts on x, y, and z axes. Print this as early in the process as possible and use digital calipers to measure how the physical object compares to the measurement settings in your digital design file.
• Use Many Layers. Get to know the layers in your application and develop a practice of duplicating all of the elements that you will use for joins, 
• Create "Reference Layers." The simple 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.
• Build a Mechanical Tolerance Plan. Anytime you use a fabrication tool to produce a physical object from a digital design, there are slight differences between the "ideal" digital model and the accuracy of the machine used to create the printed object. When you create printed parts that interface with real world objects you need to plan to accomodate this by getting to know the technical specs for your machine and job settings -- and by printing frequently. See "Creating a Dummy Object" for a great practical example.
• Create "Reference Cages." 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.) 
• Learn the keyboard commands. You will rarely use all of the modeling tools from a CAD package within one project, just a small subset. Learning the keyboard commands won't just speed up the process of modeling, it will help keep you in more intuitive creative mindset than if you are constantly stopping to mouse over to a menu item.
• Print Often, Measure Once, Twice, Thrice! Take every opportunity to print your model, or parts of your model, available to you. You will learn quite a bit about your model by holding it physically in your hand. And if you use each opportunity to measure and compare your real world model to your digital one, you will make sure you detect faster whether any of your modeling operations has shifted your design off of the baseline you established creating the dummy object.
• Print Parts of your Model. To save time and print material, it is worth creating new layers with version of your project that are trimmed down to only the element you are currently working on, so that you can export and print those. Examples include how a button and button cavity will function. 
• Adjust your model, not your job settings. While it is tempting to tweak your print job settings with lots of tiny scale and position changes in your CAM tool to just "make it work," those you share your printable model with after the design process will not benefit from what you learned about your model from printing it unless you go back into the design file and make adjustments there, upstream from your printable files.
• Fused Filament Fabrication prints shrink as they cool. And by a factor of about 1%-2%, depending on your plastic vendor. You will get the hang of this over time, but as long as you accomodate this when planning for engineering tolerances your design should remain accurate to the world.