I had to do a poster for a conference, and I came across a web page full of help and advice on how to produce a poster using LaTeX. It was actually pretty good. The layout is pretty much done for you already, if you use one of the templates (landscape or portrait). You just need to do a bit of fine-tuning when you put your own stuff on it to get the right vertical spacing.
There are also links to templates on other sites. As if that wasn’t enough, it also has tips on previewing and printing the poster, which are useful, but note that if you have created a poster in PDF format, you probably don’t need to worry about the orientation; I think this is just a problem with postscript files. That is to say, I had a landscape poster as a PDF file, and it printed out with no problems to an A0 printer. Admittedly, it was the man in the print centre that did the printing, but I’m sure I’d have been charged extra if it hadn’t come out right first time!