For lawyers, your fee is as important as the service provided. Therefore, you must understand and use the following rights: account stated, retaining liens and charging liens.
Account Stated
A possessory lien that attaches to all papers, securities or monies belonging to the client and is a general lien attaching to the entire balance of the account, dependent exclusively of possession. Be advised that they attach only to the client's money or property and not to funds placed in escrow for third parties (unless the escrow agreement specifically authorizes payment of attorneys' fees out of escrow). This can be used to compel professional charges whether related to the possessed property or not.
Don't forget about Judiciary Law 475. It attaches to the client's cause of action if and when an action, special or other proceeding is commenced. It attaches to a verdict, report, determination, decision, judgment, or final order in your client's favor. It was created to save the attorney's rights if she was unable to get possession of client's money or property. A discharged lawyer retains a charging lien but its limited to quantum meruit. This lien is statutory and equitable in nature and courts use wide discretion to decide what is fair and reasonable.
Informed consent letters and descriptive billing practices are good measures to avoid needing the above enforcement tools. Its good practice, and clients will be less apt to avoid payment and less apt to proactively sue or counterclaim against you to pressure you to back off.