In the same way as you would when planning a lesson, decide on your learning objectives. Keep them few and simple. You will have at most 10 minutes to fill, and you'll have to spend some time giving out information.
A well-run school will have a schedule of assembly subjects related to events in the school calendar - supporting the Summer fete, sharpening study skills to support a revision programme - but it may also include opportunities for delivering parts of the citizenship and PSHE curricula.
What you choose is less important than the ways in which you help your pupils build a positive group identity, and in making the values and purposes of the school their own. Assemblies, religious or not, are important in supporting the social, moral and spiritual growth of your pupils.
You'll need to work on your presentation skills. Use music to set the mood, and have it playing as pupils come in. Turning it off is a clear signal to even the largest groups that you're about to begin. Don't be afraid to use new technologies - a laptop and a portable projector can produce images that everyone in a large hall can see, and can let you share content from any medium.
Assembly rooms are bigger than classrooms. Will everyone be able to hear you? What about lines of sight - will everyone be able to see you? If you have visual aids, are they big enough to be visible from a distance? Eye contact is especially important when you're working with large groups, so pick three or four pupils in different parts of the room, and make eye contact with each of them in turn. You'll look engaged with the audience.
Aim to involve your audience. If the core of your assembly is a story, begin by asking questions to help pupils focus on its subject, and remember that stories are better told than read. Master the bones of the story then improvise around that structure. What you say will sound more personal and convincing. Remember to speak more slowly than usual, too. Give your words time to sink in.
Make pupils work. If one of your learning objectives is to get them to examine and to change their views on an issue, begin by taking a vote to establish what they think before your presentation, and take another after it. You might even use a mini-debate between two well-prepared pupils to help you make your point. Borrow formats and ideas from performing arts and television, such as hot seating, news reports, quiz shows.
Use your tutor group or class to research and deliver their own presentation, perhaps as a short play or interview. It's a good use of tutorial time, and will involve the performers in real learning.
Relate the subject of your assembly to pupils' own experience - take news items as your starting point, or situations from soap opera (or, even better, from The Simpsons). Help them take what they know and encourage them to think about it, and reflect on their own experience.
Use outside speakers. All the public services will be only too happy to visit you. Firemen and police dog handlers always go down well. Mobilise parents. They have a surprising range of skills and experience, so use them, too.
Finally, remember that pupils will be going off to lessons after you've finished with them, so don't over-excite them. Your colleagues won't thank you if they have to spend too much of the lesson to calm them down. End your assembly with a couple of minutes for reflection.