There are basic rules for making a poster not terrible, but it's considerably more complicated to take it to good or great.
One of the reasons for this is that there are many more ways to do this. You can add color, reorganize, add images, or so many things. There are some really inventive things people do with posters. There is only one real rule for making a poster better.
Rule #1: Keep your ideas the focus.
Don't add clutter (tangetially related images, too much color, too much text, etc); you need to make sure that what you add helps people better understand your ideas. But at the same time, make sure what you have enough information to keep your ideas clear.
Let look at our examples
Color and images are great to have, it really catches the eye, but that's not always good. Add some color, add some images, but make sure that they help you. In the right poster, the colors make the poster harder to read, and don't help with anything. In the poster on the left, there isn't much color, but they are all on a unified scheme, which is generally more appealing than a random bunch of colors.
Be prepared for this to take time. You don't acheive greatness quickly.