Personally, I like Gartner's definition on what constitutes SOA, which I think sums up the essentials of SOA design:

Gartner defines the SOA style as having five qualities: modular (these modular pieces are referred to as services), distributable, describable, sharable and loosely coupled. These last two items  – sharable, sometimes called reusable, and loosely coupled - are generally  cited as the key difference between SOA and other styles of system design.

That's from our Knowledge Network definition.

http://www.itbusinessedge.com/cm/docs/1184

I suspect, however, you're asking about SOA design patterns, and that's a way more complicated topic. There are books on it. You might want to look here:

http://www.soapatterns.com/

and here:

http://www.soapatterns.org/

If you're talking about how to get started in building a SOA, there's also a large body of work available. Off the top of my head, Todd Biske's blog or book on SOA Governance might prove helpful.

http://www.biske.com/blog/

http://www.amazon.com/SOA-Governance-Todd-Biske/dp/1847195865