When making a poster there are typically three common mistakes:
You have too much text
The used fond-size is too small
The poster has the wrong size compared to the guidelines from the scientific meeting
It is very important on a poster to include as little text as possible while still getting the message across. This means in practice, that you have to think graphically and include good figures and tables rather than text. As a rule of thumb you can easily use your text abstract from the scientific article and then expand it here and there to fit the poster presentation. You will supplement the text with figures and tables to make the poster readable and interesting.
A poster should be readable typically 1-2 meters from the board, and this means that the letters must be sufficiently large. Always use bold text and big letters. It is very embarrassing if you come to the congress with a poster having the wrong size so therefore check thoroughly the instructions from the congress.
Most posters are printed on large pieces of thick paper but a smart advice is actually to put it on fabric instead. You can fold the fabric in your suitcase and it is very easy to handle and travel with. Most professional printers nowadays can print on fabric instead of paper.