Talk with your advisor. There is no replacing the expertise these professionals bring.
Do your research. Don't just take the word of some Web site, look up each school you're interested in and verify the information about their program.
Pay attention to the rankings of the programs you're interested in. Be sure you know how these rankings will affect your future.
Understand the prerequisites. Usually, taking specific classes is more important than the major you choose. Know what classes will be required of you so you can wisely choose your schedule each semester.
Learn about the faculty associated with each program and their areas of research.
Talk to students in the programs. They'll be able to provide you with an inside look at what it's like being in each program.
Graduate School Tips
Talk with professors in the disciplines you're considering. Tell them about your interests and ask them about schools that they would recommend.
Make contacts. You'll need references from professors you've worked with and, if you're pursuing a Ph.D., you'll need a professor willing to take you under his/her care. Take opportunities to network and meet new people in the fields you are pursuing.
Have a plan "B." Make sure you know what you'd like to do in case your first choice isn't possible.
Go to special events like the Graduate School Fair.