You probably have done a lot of this before you had your phone interview, but you must make sure that you thoroughly review the company's website. This review should include detailed research into the company's products/services as well as review of the website's News and Press Release sections.
In addition, please try to obtain from the company a list of who you will be meeting at the interview. If you can get this list, then review each of the interviewers' LinkedIn profiles (if each interviewer has one). You want to make sure that you review each interviewers' job title, duties, previous experience, and education. Also, look to see if you have any common connections with each interviewer. All this serves as an "ice breaker" to open up the conversation. Also, you never know that a common connection may be able to serve as a strong reference for you. Any edge in the interview is a help!
You also may be asked by the company to complete an online application prior to your onsite interview. Make sure that you do this.
Prior to going for the interview, make sure that you discuss with your recruiting contact what is the best attire to wear for the interview. My advice is always over-dress for the interview even if the company's work place is business casual. However, it is best to check first with your recruiting contact just to make sure.
It may seem simple, but you need to make sure that you know the directions to the interview; especially if you have never been to the company or area before. You may be leaving your current job to go the onsite interview, so things may be a bit frantic. Also, make sure to leave in plenty of time in case traffic is a problem. If you do begin to run late, make sure that you call ahead to inform your recruiting contact. If you call ahead and inform that person that you are running late, you will not be penalized. Try to arrive about 15 minutes ahead of your interview, so you can "freshen up" before the interview.
Most companies will want you to meet a number of people in your onsite interview. The list may range from management to peers. The interview may last all day or part of one day (depending of the level of the role). Most companies have a "cut-off" in the interview. For example, if the interview is not going well, the company may let the person go before lunch (if the interview started in the morning). If an interview is going well, a lunch meeting may be included along with more interviews in the afternoon. The process may be grueling, but it is a great sign if you are kept a long time! Companies will want you to meet as many people as possible, because most companies value the feedback that they receive from both management and peers.
Just remember, a lot of the interviewers may not be all that experienced in interviewing candidates, so make sure that you are engaging in the interview. Ask them questions about their job, how they came to work at the company, what keeps them at the company, etc. Turn the tables and interview them! At the same time that they are interviewing you to be a great fit, so should you be interviewing them to see if the company is a great fit for you.
Make sure to get business cards from everyone and send a quick thank you interview to each interviewers. It does not hurt to do this and certainly may be the difference to being hired or not being hired.
If you have not heard from your recruiting contact within 48 hours regarding feedback from your interview, don't hesitate to reach out. A delay in receiving feedback may not necessarily be a bad thing as hiring managers do travel or the company may not have finished the interview process. You should have had some sense of the timing of the decision after you interviewed onsite.
I believe that by following these practices, you should greatly improve the likelihood of receiving offers. Best of luck in that next interview!