Hi Jamie,
If you don’t need the http version accessible and want to force the https you could simply redirect all traffic to the secure site with a 301, transferring all your pagerank to the main site.
If you need both versions of the site accessible, for instance if you only needed https for logged in users, and you only want one version to appear in SERPs the best thing would be to use a canonical tag to consolidate all that SEO juice into the version you wish to rank.
If there’s only a few secure pages with links to other non-secure pages then meta robots noindex,follow would work well, since the SEO juice will flow through those noindexed page and into the rest of your site, but if the whole site is duplicated on both versions this could be a big mistake.
No-indexing an entire https version would be a bad move even if you were using noindex,follow since your internal linking will be to the secure pages. Even though pagerank will be passed through those pages it will eventually come to a dead end or leave through an external links. With the canonical tag, any links pointing to your secure version will pass their SEO juice to the non-secure site, rather than be lost in the noindexed site where it has nowhere to go.
Have a little read of this interview with Matt Cutts a few years back for further clarification, it’s got a good quote about how PR flows through noindexed, followed pages: http://www.stonetemple.com/articles/interview-matt-cutts.shtml
Matt Cutts: A NoIndex page can accumulate PageRank, because the links are still followed outwards from a NoIndex page.
Eric Enge: So, it can accumulate and pass PageRank.
Matt Cutts: Right, and it will still accumulate PageRank, but it won't be showing in our Index. So, I wouldn't make a NoIndex page that itself is a dead end. You can make a NoIndex page that has links to lots of other pages.
So it’ll be different depending on your circumstances but if you’re in doubt, the canonical tag is your best bet as you’re only consolidating those pages in googles eyes. If those pages perform well and you noindex them without sending that PR somewhere useful you could be throwing away all that benefit.
Hope that helps,
Tom