Interact, perhaps, with the firm's twitter profile.
Say thank you to your interviewer at the end and after by email or send a 'thank you' letter by post.
Put on a nice suit. Be smart and neat.
Stipulate the goals of your career and the reasons you prefer the position.
Do not use unprofessional words.
Think of honest questions to demonstrate you're curious and have researched.
Advice for Job-hunters:
"Read a paper or relevant blog that writes about the niche you're looking for a job in. This will make sure that you're up to date about movements in the field and will also be useful in an interview and shows the possible company that you want this job."