My suggestions:
Put together a very good, detailed ad that fully describes the car. I don't think you can put too much detail into the ad (as long as it's relevant and true ;-)).
Take a LOT of detailed pictures. If you have access to a friend or even a pro or semi-pro photographer consider using one.
Don't be afraid to ask for good money for a top-condition car. Especially if you don't HAVE to sell. There are always buyers for super low-mileage cars, although you might have to be patient in this economy.
Here are the places where I would advertise:
Local Craigslist and possibly other Craigslists within a reasonable distance of your location. Include links to your pictures either in the Craigslist ad(s), or to a website where the pictures are located. Note: Craigslist ads expire on a weekly basis, so you have to keep renewing them, which is kind of a pain, but I think it's worth it.
Online sites including: Pelican Porsches for Sale, Rennlist Classified (you have to be a member, but it's inexpensive), the PCA site (both online and in Panorama--you have to be a PCA member), PML (Porsche Market Letter) as recommended by Techweenie.
Additionally I recommend buying a non-expiring Auto-trader ad. Some Autotrader areas don't allow much of a description and their picture hosting is kind of crappy, so be sure to include a link to all the great pix you have taken.
There is one other site that I have used very successfully in the past for two ultra-low-mileage cars that I had. I think they are still in business:
www.carshowing.com. I don't know what their current charges or procedures are, but these two cars both sold almost immediately after I posted them there. No doubt a result of my brilliant copywriting --
Here are the links to those two cars as an example of how I would do it. This site has a lot of "regular" cars, but I discovered it when I ran across a really nice 914-6 (conversion) that was advertised there. So they do somehow reach the enthusiast market--or at least they used to.
951 with 21K miles:
http://www.carshowing.com/detail-sold.php?fatherID=36&ListingID=18&TypeID=42
944S2 with 30K miles:
http://www.carshowing.com/detail-sold.php?fatherID=36&ListingID=12&TypeID=42
Hope this helps and good luck with the sale.