What is the best method of customizing a site using RETS?

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mdean already outlined the steps that you need to take, basically. I'll add a bit because we were in the same boat...

We primarily use wordpress, but weren't satisfied with the plugins that were out there, or the costing associated. My clients typically want branded and highly customized high end looks, and not some sort of iframe thing that obviously doesn't match the site.

I'll compliment mdean's great overview with my actual experience, having built a MLS and RETS library, etc., and implemented it on a couple websites:

• I ended up using phrets, because wordpress is php too; It facilitates most of the RETS functions you need, and is a handy library.
• contact your regions MLS. Mine had some good resources to get me started.
• get RETS Connector. It's free, and is REALLY handy for building your application (e.g. looking at data and metadata).
• Unless you have a unique situation, be aware that a "RETS feed" doesn't mean you query against them in realtime. You most probably need to download a complete copy of the data you need to your own database. mdean indicates that, but most docs I found didn't make that perfectly clear. So, for my MLS situation, I download a complete set of data once a week, and updates once an hour [RETS makes getting updates easy, however].
• Because you're storing the data, you will need to build and maintain a database, which means translating their dictionary to your own database. I wrote functions that takes only the classes and items i want to keep data load time/size down, build the table, and then do further rets requests to get the data and populate my local database.

I think the biggest takeaway I will give you is that the RETS feed is what you need for what you want, and get complete customization (excepting your local MLS requirements like displaying a broker reciprocity logo, etc.). BUT the trade off is that you will need to BUILD the IDX solution. RETS simply helps you get the data, but you will be in charge of grabbing it, maintaining it, and writing code to display it. It will be a fair amount of work; it's not just get the feed and done. Worth it, if that's what you want, however!

I know this post is old but I’m having trouble finding out the Broker Reciprocity requirements. I have reached out to the local MLS and I can’t get a response back. Every broker in the MLS area all have the disclaimer and logo but I don’t know how to obtain this and use it properly. I am building this out for a broker NKY MLS.