Make sure that Poster visuals are clear and of good quality. There are different ways to do this, depending on whether it is a graphic, photograph, diagram,
Use all the space at your disposal, but do not cram in the content - white space is an important part of the layout, and good use of it can make a poster elegant and arresting.
Use colour sparingly - limited use of a few colours is more striking than a 'rainbow' approach. Think about why you are using colour; it is especially useful for emphasis and differentiation.
Avoid colour combinations that clash (e.g. red on blue) or cause problems for people with colour-blindness (e.g. red and green in proximity).
Use white or muted colour background (e.g. pastel shades)
The flow of information should be clear from the layout - if you need to use arrows, you could probably lay it out better.