1. It was Edward Hordern (my mentor) who taught me the first and, I believe, the most important principle to designing a "nice" puzzle which consists of a number of pieces e.g.. box packing, polycube assembly, dissection etc. and that is:- THE FEWER PIECES THE BETTER - provided, of course, that the puzzle is difficult to solve. Excellent examples of this principle are Stewart Coffin's three piece puzzle and Dic Sonneveld's three piece burr both of which are very difficult put-together puzzles. Another nice but easier three piece puzzle is Oskar van Deventer's "Oskar's Blocks". The ideal is not to exceed 12 pieces in your design.
There is nothing more certain to drive away members and potential members of our puzzle fraternity than to frustrate and anger them with a puzzle that is ill conceived....
Unlike Stewart's, Dic's and Oskar's your three piece design was too easy so you added a piece.... still no good, another piece, and another and yet another until you end up with 50 odd pieces that even this humble PC would find difficult to solve. So, where is your beautiful hand crafted work of art now? Probably at the back of a drawer never to see the light of day again or more than likely it has been cremated! Simply because it was not a "nice" puzzle.
Here endeth lesson one.

2. Don't allow your PC to design your puzzle for you, it has no soul!
It is also invariably bad tempered and uncooperative, and certainly does not have any sense of humour! All of which does not help with the search for the Ah! Ah! factor which is another major element in the design of a "nice" puzzle. What is the Ah! Ah! factor?
You've been putting pieces here, moving them there, pushing this and that, spinning, rotating; making every piece placement or move but the right one. Then, suddenly (perhaps with a little dose of lateral thinking) the penny drops. Ah! Ah! you say..... or if the puzzle design is really special, mind boggling even, your exclamation will be WOW! Can your PC, any PC, assist you here? I think not.
OK, I admit it. I have used my PC during the course of designing a puzzle; but merely as a tool not as a source of inspiration. In the past I produced all my drawings and leaflets by hand, now I use Corel Draw, Photoshop, Word etc. I would also claim that a polycube assembly had but one solution, now I can prove it (or not) by using Bill Cutlers BCP Box program. Tools, my workshop is full of them.