Let's assume that you're using HL7 Version 2.xx (like 99.9% of the rest of the world). And the simple answer is that you can't exchange HL7 over the internet to/from your Web Service. Don't let this get you down, though. The problem is NOT you. You can design a beautiful web service for HL7 messages that's easy to understand and post to but your problem is the same problem that everyone else has, it's your HL7 Trading Partner (the other guy, the one that you actually NEED to exchange HL7 messages with).
Since every HL7 interface is a negotiation between 2 systems, and since you're dealing with "for profit" companies here, finding ANYONE who's willing to change their system just for the privilege of exchanging HL7 messages with YOU is practically nil. Much more likely, you will encounter a Trading Partner who has written THEIR OWN beautiful HL7 web service which they would be happy if YOU would change your system to use theirs.
There IS now a solution that you can use TODAY which is compatible with ANY HL7 version 2.xx compliant HL7 system which will allow you to interface THEM to YOUR Web service. It's new and it's actually very easy to implement and it's called the UltraPort HL7 Postmaster.
Here's a link to the online help http://www.hermetechnz.com/documentation/UltraPort/FSPM/common_use_scenarios.htm. Look at Scenario #2 on that page.
Hope this helps you.