Quick and dirty, I was going to write, but the output is quite nice, actually.
First, some history: I was about the first in my department to try to compose a scientific poster with LaTeX. I basically used lots of boxes, and it was a matter of using tables to align them. A bit of a pain, since LaTeX is best when one does not want complete control on the final outcome. My poster was used a template for several posters done at my department, despite its flaws.
Recently, I was in a hurry to compose a new one, and I had lost access to my files. Fortunately, a google search for a0poster yields Andrea’s site. The tex file there is really useful. There are two options, I have just tried the one that seemed simple, using multicols. You just write stuff and it gets nicely placed in three columns. Figures (and refs to them) are supported, as are citations, even BiBTeX. If the data is already at hand (as is usually the case, e.g. from a previous paper), a nice poster can be composed in just a few hours!