Do not fill up a basin with water and detergent, throw the brushes in and leave them to soak!  Sounds tempting and easy, but doing this will not get the brushes clean and will damage the brushes as the bristles will come away from the ferules and the paint will peel from wooden handles.  Ensure you don’t used a heavily scented detergent as a little of the scent will remain on the bristles and whilst, you might not mind, your client might hate lavender!