Hi Jon
I've used this website to book ferries online (it uses a wide variety of greek ferry companies) and collect them at the port. You print off a confirmation, present it at the ticket office and they print off the tickets for you there and then. Really easy. Prices are only available from about 4-6 weeks before you want to travel....so just keep looking nearer the time. The Flying Cats are brilliant....so smooth you can hardly feel they are moving!
http://greekacom.forth-crs.gr/english/npgres.exe?PM=BR#RoutesTable
Kath

Poros is a great little island. You will probably need to book accomodation as you're visiting in August. Not sure about ferries from Athens, we got the ferry from the Peloponesse which is so frequent booking is not required

Ferry times won't be out for August for some time yet, but to get an idea based on last years schedules use this site,
http://www.openseas.gr/OPENSEAS/index_en.vm
It allows you to put in dates for 2010 so that you can see what was available then, about 10 a day in August.
I agree that booking accommodation is the most important thing, most of the Greeks (as well as Italians) will be on holiday then, places to stay become scarce on many islands & coastal towns & villages.
I've never booked a ferry much in advance, just got tickets from an agency a day or so before, or even on the day, before moving on to the next island. If you want a specific day or time though I would book purely because it's August & to give you peace of mind.
As it's Hellenic Seaways that serve that route I would book direct with them if you are doing it online,
http://www.hellenicseaways.gr/index.asp?a_id=238