Best Practice #4 is to favor REST over SOAP, except for those scenarios that demand SOAP.
When it comes to cloud computing platforms, it's a RESTful world and Azure is no exception. The vast majority of the services in the platform can only be accessed via REST (even if you're using .NET library code, REST is what's happening under the hood). SOAP isn't completely out of the picture, however. For functionality that depends on WS-* protocols (such as federated security) Azure does use SOAP.
Which should you use for your own Azure-hosted web services, REST or SOAP? It makes sense to emulate what the platform itself does and favor REST, except where you must use SOAP. Using SOAP makes sense for compatibility reasons or when you need WS-* functionality. When these conditions don't apply, a RESTful interface is recommended. It's simpler and more in line with "how we do things in the cloud."
The popularity of REST is due to its simplicity and its resource-oriented nature while SOAP has become cumbersome in the opinion of many. There is a good discussion of whether SOAP and REST are complements or competitors on David Chappell's blog.