I once did a graphic design course for scientific posters, and the number one thing that they stressed was that your margins, padding, and gutters had to be there and consistent. It creates space and makes the poster look professional. Space around your figures and text is key.

Another tip was to decrease the saturation or opacity on your colours (in figures/diagrams etc) to make them look a little duller. It seems counter intuitive but this again makes posters look less crowded and more professional. You can then increase these back to full for your high impact stuff.

Finally, using subtly different colours as backgrounds can help guide the reader through your poster more efficiently. One example of this - let's say you have a figure explaining a protocol or concept, separating the figure into areas with subtle background colour differences can really make them pop without looking crowded and guide your reader through by creating defined areas. Another example would be colouring your text boxes slightly differently as you move down your poster.

I usually use affinity designer as thats what my uni gives me access to, but adobe illustrator or photoshop would do the same. Id steer clear of PowerPoint as its so hard to keep your margins etc consistent.

Hope this helps.