The first thing is that, if they are referring to the meta description tag, they are wrong that it will help you build rank. Like the meta keyword tag, Google hasn't used it as part of their ranking for some time (years).
Best practices for them? In terms of total length, 150-161 characters, depending on search engine. Longer than that and you'll run into truncation, which is up to you if that's an issue or not. Think of it as ad copy.
Re: keywords, write them to engage the potential client. Since you should be writing a uniquely relevant description for each page, you probably don't want to be targeting 5-10 keywords for any one page as it is. If you decide to not write them at all, Google will pull an excerpt from you page, upping the need to have quality, relevant content. Depending on the site's content, I decide whether to write them. I usually recommend them because many lawn care-targeted sites that people ask to be reviewed are pretty thin on content.
I know lawn & landscape marketing, branding, and web strategy. I'm not anonymous, I just don't self-promote.