You have one glance to grab someone’s attention with a poster. High contrast between elements can help you do that. Forget a monotone color palette with pale gradients; go bold with color and type options. Poster design is a great time to try a typeface or color palette that might be too “crazy” for other projects. Experiment with it.
Think about a big color background as well. Many times poster designers start with a white canvas. If your printer allows, use a high color background with a full bleed to make your poster stand out from all the rest.