The white space in the margins can be very useful! I have two widgets in the margins of www.50bubbles.com just for example. Both are free widgets available on the web so don't think you have to be a programmer to get this stuff. They show up on every page of the site and are highly visible; which is the point.

One of the most obvious uses for margin call outs is "CONTACT US". It should be on every page of your site anyway and your visitors should not have to look for it. You might also consider posting your most recent announcements here. Current, relevant information that you want to bring attention to - "WE ARE HAVING A SALE TODAY!!!!" for example. It's also a common place for your social media links.

Best practices? Simple is better. Don't overuse it. If you fill the margins with junk you'll lose the affect. I'd recommend one call-out at most on each side.

For those of you who wonder how I added mine, you do have to drop a little bit of code on the page. (You don't have to write it, that's done for you. If you don't know how to add html to a web page, certainly your web developer will). In any case, here are links to the two margin widgets I'm using. Both are free.

Spot IM: It's on the bottom left and will fill up the left margin if you open it up. It works like phone texting but for your site. For me, I use it like Twitter. I like the content on my site. I'm hoping for some development to happen to this widget that would be useful (for example the ability to have topics.  

https://www.spot.im/

GET Site Control: The "Free Google Advertising" call-out on the right hand site is from https://getsitecontrol.com/ 

There are many widgets to be found here and the first one is again free. It's extremely easy to use. I may end up actually programming this feature into the website because I like it so much. 

So I basically provided you a means to use that margin space with very little effort and it may be just the thing you need to increase phone calls or conversions. Hope it helps!