A demonstration meeting can be one of the easiest ways to build membership. Organising such an event satisfies the requirements in the Component Leadership (CL) manual: Organise & Delegate for organising a special event, or Motivating People for running a membership campaign.
Choose a specific date in advance, and publicise that your meeting is welcoming guests. Create a flier like this DemonstrationFlier2011 or use some of the promotional materials on the Increasing Club Membership page.
Invite past members to attend.
If you have just a one hour meeting, consider having just one speech & evaluation or take out optional roles such as jokes/tips/invocations to keep the meeting simple and allow time to talk about joining the club and have questions and answers.
Perhaps invite a special speaker, or have some food/drinks available. If you don't have enough active and available club members to fill all the meeting roles, ask your Area Governor or another club to send some volunteers to help.
Put on a good show!
Show the guests how a Toastmasters meeting works, and explain all the roles well. You can provide a handout or promotional materials for guests. And don’t forget to ask guests to join!