Depends on what the topic is and for what grade level. Most important is accuracy and the second piece is creativity. Put those two together and it is a guaranteed "A". Accuracy is number one. Creativity...go 3D, use things you collect inside/outside the house, depending on deadline use food products, etc.
The one science fair project I did as a kid that really stuck with me was creating a giant electrical circuit on a poster board. You get light bulbs, switches, and batteries, do some research on basic circuits, lay everything out on the board and connect with wires. Some questions to ask are: "What makes the light bulb shine?", "How/Why do switches work?", "What are electrons and how do they move throughout the circuit?". Be creative and encourage experimenting. Failure is good in science. Why something doesn't work is often more informative than why something does work.
on a poster board for any project children are always wondering how to make it look good and neat to gert a good grade on it but the worry here is solved by stenciling in neat letters and printing out paragraph information then cut it neatly and glue to the board