#Protocols:
By default your web services should communicate via HTTP over SSL. You should have a pretty good reason for diverging from that.
There are two main options of protocols to use for your API, REST and SOAP.
##REST A protocol that reuses the HTTP verbs and describes resources by URLs. (Simple, right?) For more information, check out wikipedia.
##SOAP An XML based protocol often found in enterprise/banking applications. For more information including a list of advantages/disadvantages, check wikipedia
Most modern APIs are adhering to a RESTful format, rather than using SOAP. With that in mind, REST may be a more cofortable default protocol.
Both REST and SOAP should be well supported by clients.