I use the tri-fold project board on a trapezoid table to display and organize my math centers (special thanks to my friend and colleague Jenny Neville for helping me come up with the project board math center idea). It not only helps to "advertise" new center activities, but it is excellent for helping them find a center activity that focuses on a particular skill, or an activity that offers the level of challenge that they need. In each of the classrooms where I teach, I stapled nine colored file folders onto the project board in three rows of the three. Any cards, manipulatives, or other materials that accompany the activities are placed on the table in front of the board.
Each row is a different challenge level. For my math centers, I used traffic light colors:
Red row (top) -- Challenge / Enrichment activities
Yellow row (middle) -- Practice activities
Green row (bottom) -- Review activities
This allows for student-driven differentiation if your students are ready to handle the responsibility of choosing activities for themselves. You could always assign certain students to particular color levels if you felt they need more direction. Sometimes I will personally select an activity for a student, either because I feel that he or she is choosing something too easy or too hard, or because I know that a student needs practice on a certain skill. However, my students are typically very good at selecting work for themselves.
Another good organizational tool is a bulletin board display. It can be set up the same way as the project board, but with the potential for more available topics and difficulty levels if desired. Like the project board, it is also highly visible and inviting, and it helps to advertise new activities for you. It's also one less bulletin board to have to decorate over and over again. :) The drawback is that you don't have the table for manipulatives and other materials that don't fit into the pockets, although placing a slim table or row of extra desks in front of it or beside it might solve that problem.
The storage crate is another option for organizing your math centers if you have an extreme lack of space. Any centers, provided they are filled with worthwhile activities, are better than nothing. You can still color code your folders to help organize by topic and/or ability level. However, the storage crate is not highly visible, and does not help students to see their options of activities very well. It is also difficult for more than one student to dig through the files at once.
Hopefully these ideas will be helpful in setting up and organizing a learning center in your classroom.