PCB Design & Debugging Strategy
For microcontrollers, a user switch, a user LED and a TTL-level serial port brought out to a header can be useful, especially if there are no other simple communication methods (maybe there's just an Ethernet port, which might not work right off).  

If you put no other test points in, at least put a ground test point of adequate size for an oscilloscope probe. 

Almost all of the things you will do to ensure adequate testing in production will help with debugging. Generally speaking, designing the testing (including fixtures and procedures as well as features on the board) is probably a similar amount of work to designing the board itself.