1. Ask employees how they feel about the union.
2. Announce that the company will not deal with the union.
3. Tell an employee that they will fire or discipline them for union activity.
4. Threaten to close the business if a union is voted in.
5. Say that unionization will cause the them to lay off people.
Read the rest of the "35 Things Your Employer May Not Do!".
It is your RIGHT to choose to join a labor union. The exact text of Section 7 of the National Labor Relations Act is copied below:
RIGHTS OF EMPLOYEES
Sec. 7. [§ 157.] Employees shall have the right to self-organization, to form, join, or assist labor organizations, to bargain collectively through representatives of their own choosing, and to engage in other concerted activities for the purpose of collective bargaining or other mutual aid or protection, and shall also have the right to refrain from any or all such activities except to the extent that such right may be affected by an agreement requiring membership in a labor organization as a condition of employment as authorized in section 8(a)(3) [section 158(a)(3) of this title].