Enter Portbooker.com. A recent import that has taken European marinas by storm.
Portbooker helps you reserve slip space based on your boat's LOA at marinas across the U.S. and Europe. (Though, one downside is that you'll need to know your boat's LOA in meters.) You can compare other users' reviews to find what you're looking for in a harbor. Though not all marinas have joined with the booking program, most are listed in Portbooker's directory.
As part of their U.S. debut, a charter raffle is being held of all new members (free to join!). Everyone who signs up by the end of the year will be entered in a raffle to win a week-long charter. Two winners will be chosen: one for a Med charter and one for a charter in the Caribbean (St Lucia) aboard a 10 or 12-meter boat.
Let us know if you try it or if you have any other ideas for locating a slip in a new port.