So what is the perfect handshake? Most believe it should be firm, but not crushing. Too weak and you can come across as lacking confidence; too strong, and you can be seen as over dominant. Typically 3 or 4 "shakes" are acceptable, and make sure you look your interviewer in the eye and smile. Try to make sure your hands are clean and not sweaty and that your fingernails are clean!
For something that lasts only 2 or 3 seconds it can make all the difference in the world to the first impression.