How to promote multiple sites?

I would really appreciate some advice on this.

Build back links to them separately works better.

This will allow each site to stand independently on its own and any sharp change in Google algo might not affect all your sites/earnings.

With 10 sites, you're spreading yourself too thin. Why not focus on just one, promote and grow it, and then build more later?

OP, don't interlink them.

Instead, do a major silo structure on each one, and then create a regular internal link structure - each new content should link to another page on THAT site, preferably a "money" page.

As for the rest, keep on doing it, especially the "other people run them". Some people here assume weird stuff but they forgot to ask you how many niches are these sites, and how many sub-niches are they... and so on - which is a clever and profitable way to profit from one, two or a dozen niches.

Plus, you haven't got all your eggs in one basket.

I agree with the posters saying that ten sites is too many. Perhaps you could combine those into 2-3 really good sites? Building that many backlinks is soo time consuming. If you do go with the ten site plan, I would suggest building backlinks individually. You can also try different strategies with the different sites and see which one is the most successful.

I think what the OP is saying is that he hosts 10 sites which are owned by clients of his, and he's contracted to promote them.

In that case, interlinking between the sites is not an option.

There are back-link building programs (software/services) where you can add multiple URL's and the links are split somewhat evenly across each of the (10) URL's. But to be honest, I'd stay well clear of any/all of these 'link building services'.

You could go to Fiverr and order 10 link wheels/pyramids for $50, but again, I'd stay well clear of doing that unless you want to damage your client sites.

Others will disagree with me, but IMHO you are stuck doing what you are doing the way you are doing it. You can take shortcuts as mentioned above, but if your reputation is worth something to you, I'd advise against it.
Keep developing relationships with other sites. Approach sites in a similar niche and ask if you can make guest-posts or contribute to their blogs/forums.
If you create a template for your link building activity there's no reason you can't outsource it via Elance. But give your contractor very clear instructions on what you want from them and check their work periodically.
And yes, make sure you have a good internal link structure on each site, both from the navigation and the use of anchor text links in content. And encourage your clients to add content frequently. Ensure they provide easy mechanisms for people to 'share' their experience via FB and other SM sites. That way links can be built more organically, which is the right way to build links.