GPA
While you should keep a respectable GPA (3 or better) there is little evidence that schools are overly impressed with a 4.0. It may be better to use some of the time preparing for the MCATs (see below).

What courses should you take?
Most medical schools have a set of minimum requirements, but these are usually automatically covered in the Biology degree. Upper level courses that are usually recommended vary from school to school. In a survey of over 40 medical schools and pre-med programs, the following were specifically suggested:

The MCAT is a comprehensive exam that tests you in four areas: Verbal reasoning, Physics and Chemistry, Essay writing and Biology. While your undergraduate classes will give you most of what you need for the MCATs you should plan to invest a considerable amount of time preparing for them. You will need to be familiar with chemistry through organic/biochemistry, as well as the full range of physics. Do not expect to do well without spending a lot of time studying these areas. The Kaplan courses are recommended by many students but they are expensive and still require you to do the work on your own.

Medical/Volunteer work
Most medical schools like to see evidence that you have sufficient interest in medicine to do some related activities as an undergraduate.

Research
More schools are looking on undergraduate research as an added bonus. While not required, it will get you additional scrutiny in many cases. These sites have a lot of information, but we do not guarantee their value:

http://www.medicalstudent.net
http://web.mit.edu/amsa/www/guide.html