Catalog your library. Record every book on your shelves either on paper or in an electronic spreadsheet. Write down the title of the book and the author. If you own several editions of the same book, record the edition or publishing date.
Number duplicate books by either writing inside the jacket cover or on a small square of paper, which you can then tape to the front, back or side of the book with tape that comes off easily.
Create a sign-out sheet. When students borrow a book, have them sign their name on the sheet to prove that they still have it. When they return the book, have them sign again to prove that they have brought the book back. In lieu of a sign-out sheet, create sign-out cards that students place in pockets on a bulletin board and that they can only remove upon returning the book.
Establish some lending terms for your spreadsheet. For example, if a book is on loan, record "Out," and if it is on the shelf, write "Available." If a book is missing, record "Missing" or "Lost."