Sign your check legibly, and use the same signature that they have on file at the bank.
If you like, you can also write a memo (your checks may say "For" instead of "Memo"). You might want a reminder of when and why you made this payment. The memo can help you with that. In addition, your service providers might ask that you write something there. For example, they may ask that you include your account number in this space.
Do You Even Need to Write Checks?