If you are planning to attend grad school, you will probably need letters of recommendation. There are several things that will help you get a good reference.
Give your professor plenty of time to write the letter. A minimum of 2 weeks is recommended.
Give your professor a transcript. This will help them descibe your academic ability.
Give your professor complete instructions in writing about deadlines, addresses and contact information about who the letter goes to.
It is more professional to write a letter requesting a recommendation than to stop a professor in the hallway, or just drop some forms off in their box. A sample of what to include in this letter can be downloaded here.
Attending grad fairs and/or visiting the schools you are interested in will help you apply.
PS. Dont forget to send thank you cards!