Credentials: Your admission to graduate or professional schools will likely be based on your grades, your recommendations, and your scores on the appropriate standardized exam for your field (e.g., the Graduate Record Exam (GRE) for a liberal arts program, or the GMAT exam for a business school). It may not be necessary to have strong credentials with respect to all of these, but it's likely you need strength in at least one of them. It's also likely that you need a stronger application to obtain financial assistance than otherwise.

Taking standardized exam: If you're applying for financial assistance, the appropriate standardized exam generally should be taken during the fall prior to your intended start of graduate/professional education. This is because many programs start to decide which students will be offered financial assistance by December or January.
Links with more information concerning standardized exams:
Dental Admission Test (DAT)
Graduate Management Admission Test (for business programs, including Management of Information Systems)
Graduate Record Examinations (for most liberal arts programs, including computer science)
Medical College Admission Test (MCAT)
Pharmacy College Admission Test (PCAT)
Optometry Admission Test (OAT)

Don't let fear of cost prevent you from applying. Many graduate and professional students have financial assistance, which may include both a tuition waiver and a modest income (in return for which you may be a teaching assistant, lab supervisor, research assistant, etc). Don't decide that graduate or professional school is too expensive for you until you have found out what your real cost of living is likely to be.

Investigate programs. This is easier today than it was when the author was applying to grad schools, due to the existence of the Web. It may happen that a department or school that isn't top-rated overall has an excellent program in your area of interest, or, conversely, that a highly-rated department or school is weak in your area of interest.

Apply to at least one large program, and/or at least one program near you. Many students chose to attend a small school. However, a small school is often a little-known school, particularly outside its geographical area (that's not a statement about the quality of its education). Unless the admissions committee knows of your school or a member of your major's department, it's likely that your application will be regarded as a "gamble." A program that only admits a small number of new students each year is less likely to gamble on an admissions decision than a larger program.

Will an employer pay for graduate/professional education? Some will. If you think you want to work and continue your education, explore this question with potential employers. Your additional education may well be an asset to your employer, so this is a question well worth investigating. Because this would be an investment by the employer in your future, there may be a requirement that you promise in turn either to remain with the employer for a period of time or refund the employer's expenses should you leave.

What about my relationship? If you're married, engaged, or just "going steady," your personal life is an important part of your life, and should quite properly affect your planning. It could well make a difference where you apply, what kind of financial assistance you need, and under what circumstances you can accept admission. You probably need to coordinate your plans with those of your significant other - it may be necessary for that person to find suitable employment, admission, or other activity nearby.