Keep a reference chart on hand that includes all the commonly used proofreader's marks. This is particularly handy when you're first learning. You can purchase these charts from bookstores or find them online.
Always choose a red or blue pencil for proofing. These marks will stand out and are easy to erase as well. Avoid using pens if possible. If you do use a pen, use red or blue ink and make sure it doesn't smear.
Always try to get a copy of the work you're proofing with plenty of margin space. This makes it easier for you to place marks on the work you're editing. Proofreader marks consist of a combination of symbols and abbreviations that can be difficult to use if you don't have extra margin to work with.
Be consistent with your marks. Some proofreaders will alternate between using abbreviations and making notes in the margin. If you begin using marks, stay with the marks throughout the document rather than using them part of the time and notes in the margin at other times. Use proof symbols when possible. Reserve comments and directions in the margins when you're proofing for someone who is not familiar with proofing marks.
Take some time to practice your proofreading marks. This will help you develop a natural way of writing them and can also help you write them quickly and with fluidity. If you do a lot of proofreading and editing, the ability to make marks quickly without giving them much thought can help speed up the process. Practicing your marks will also help you develop a neater approach, which can make it easier for the clients you proofread for to read your marks.