Do not overstate your qualifications, academic performance, background, length of employment, etc. The most important things are to be yourself, be confident in your educational preparation and experience, and have your career goals clearly defined.
Every person you meet during the course of the day is a potential evaluator. This includes the parking attendant, security guard, administrative assistant, server at lunch; be aware of your verbal and non-verbal communication.
Make an effort to talk more deliberately and articulately than usual. Don’t use the words “I think,” “I guess” or “I feel” which sound indecisive. Avoid phrases like “pretty good” or “fairly well”. Avoid constant use of filler words like “you know,” “right,” “like,” etc. Use positive words to describe your skills. Maintain reasonable eye contact and be aware of negative body language such as crossing your arms or slouching.
If there is a chance that a potentially detrimental issue could arise such as a below average performance appraisal from a supervisor or poor academic grades one semester, you should have an answer prepared that puts the issue in a positive light. Never apologize for any shortcomings and avoid any comment that can be construed negatively. For example, you might explain that you had an unusually heavy study load that semester, that the experience helped you to improve your study skills and that you subsequently pulled up your grade point average the following semester and have maintained good grades ever since. The same goes for discussing former employers; keep it positive, and never say negative things about a previous boss or position, no matter how bad the experience was.