Be gracious: Always say thank you at the end of an interview and follow up with a thank you email or note.  Someone took the time to meet with you, learn about you, and consider you for a role.  Being gracious shows that you don’t take that time or effort for granted.  Even though it seems like a rough process, the recruiting process (on the company side) takes a lot of effort and coordination.  Not only thank your interviewer, but also anyone that helped you schedule it, offered you water, greeted you at the front desk, etc.