There are many things to consider when it comes to printing as everyone will tell you. The resolution of 300ppi is pretty much standard as most printer's machine is set up for that. However that does not mean the actual print will have 300 dpi. All depends on how large of a poster you wanted. Bearing in mind the larger the print, the further away will the audience stand to get a full view of it. Other things to consider of course is colour matching, which is also the most difficult part to achieve in printing. You will need to talk to your printer, say offset printing, to get a PPF of the machine they intend to use. This file contains the information of that machinery regarding all complicated setting and you can download most of this PPF from Adobe site. Also ask them about the colour management settings and they should be able to advise you. Ok, may be I am boring you now. The last thing to consider is file format, PDF is most popular, then EPS, handy but make sure you have all your linked files in one folder and properly named ( keep the spot colour on a seperate channel ). Oh don't forget to use outline for your fonts if there is anyway if you are using PDF file. Hope this helps.........