Authenticate with Custom Activities that Consume SOAP Services with Custom Types?

I’ve successfully created custom activities for numerous methods for two different SOAP services. Authentication is required using Windows authentication and many of the methods use a custom type.

When I attempted to use my new custom activities, there is no means of authentication. I get unauthorized errors, as the services do not allow anonymous access.

I can authenticate with HTTP, but it doesn’t seem to last more than the one HTTP request. I cannot use SOAP activities because of the custom types, which just cause odd errors. How can I get these custom activities to work? And if I can’t, is there a way to consume this service otherwise?