Electronic component selection while designing

What are the general considerations need to take care while selecting electronic components in design?

This is to general question.
It depends on the design, requirements... You have to see what are power supply, cost, temperature, signal BW, loses, EMI............... There are so many considerations and all depends on the design.

My ques is more related to EMC prevention, specially in power supply section. Design is based on TNY264 SMPS controller.