Looking for a Top Ten Design Tips for 3D Prints to share with teachers to use with students for each level when creating self made projects. We are using Tinkercad and Photoshop with a Tinkerine DittoPro printer.
Here’s my top 5 (or more correctly the first 5 that came to mind).
- KISS - too much detail (at first anyway) will result in too much trouble shooting.
- Orientation - sometimes rotating a shape will result in much less support, much stronger design.
- A 5mm peg will not fit into a 5mm hole. Design must account for manufacturing tolerances.
- Plagiarize - well not really, but learn from others mistakes. Lots of neat shapes and designs on sites like thingiverse and youmagine
- Related to (1) - start with models, toys and trinkets. Don’t do something structural until you have a feel for the basics.
Orientation matters for strength. The x-y plane will always be stronger than the z plane. Look at your designs and determine what might happen if forces are applied that would pull the layers apart.
The z axis can provide higher detail than the x-y. There is a minimum radius of feature size that is determined by the nozzle diameter. You cannot have a perfect 90 degree corner, or sharp features. Because of this, any features smaller than the diameter of the extrusion will not exist in the print.
Make sure the teachers get their feet wet first. 
I like @Alex_Wiebe s list. Perhaps you let the kids experience #2 and 3.
@Alex_Wiebe With regard to your no.3, I find that about 0.2mm clearance is a tight fit, almost locking, and about 0.4mm is a smooth, removable fit.
Clearances along the z-axis tend to grab a bit more too, because the layers make slightly corrugated surfaces.
Clearances along the x+y axes tend to be very smooth fitting, and allow easy sliding if you are designing a fit that you want to come apart later.
Thank you for all the great tips! Keep them coming!