15 U.S.C. § 1060 governs assignments. Among other things, § 1060 requires that assignments be in writing:
§ 1060. Assignment
(a) (1) A registered mark or a mark for which an application to register has been filed shall be assignable with the good will of the business in which the mark is used, or with that part of the good will of the business connected with the use of and symbolized by the mark. Notwithstanding the preceding sentence, no application to register a mark under section 1(b) [15 USCS § 1051(b)] shall be assignable prior to the filing of an amendment under section 1(c) [15 USCS § 1051(c)] to bring the application into conformity with section 1(a) [15 USCS § 1051(a)] or the filing of the verified statement of use under section 1(d) [15 USCS § 1051(d)], except for an assignment to a successor to the business of the applicant, or portion thereof, to which the mark pertains, if that business is ongoing and existing.
(2) In any assignment authorized by this section, it shall not be necessary to include the good will of the business connected with the use of and symbolized by any other mark used in the business or by the name or style under which the business is conducted.
(3) Assignments shall be by instruments in writing duly executed. Acknowledgment shall be prima facie evidence of the execution of an assignment, and when the prescribed information reporting the assignment is recorded in the United States Patent and Trademark Office, the record shall be prima facie evidence of execution.
(4) An assignment shall be void against any subsequent purchaser for valuable consideration without notice, unless the prescribed information reporting the assignment is recorded in the United States Patent and Trademark Office within 3 months after the date of the assignment or prior to the subsequent purchase.
(5) The United States Patent and Trademark Office shall maintain a record of information on assignments, in such form as may be prescribed by the Director.(b) An assignee not domiciled in the United States may designate by a document filed in the United States Patent and Trademark Office the name and address of a person resident in the United States on whom may be served notices or process in proceedings affecting the mark. Such notices or process may be served upon the person so designated by leaving with that person or mailing to that person a copy thereof at the address specified in the last designation so filed. If the person so designated cannot be found at the address given in the last designation, or if the assignee does not designate by a document filed in the United States Patent and Trademark Office the name and address of a person resident in the United States on whom may be served notices or process in proceedings affecting the mark, such notices or process may be served upon the Director.
Depending on the particular circumstances at hand, assignments can range from simple to very complex. If you can afford it, retain a trademark attorney to assist you in preparing and negotiating assignment documents.