This week Rand has some tips for when it’s appropriate to link between your own sites, and when it’s not.
Link for humans/user first; engines second
While you’re going to want good anchor text to flow link juice from powerful pages to ones that need indexed, think also about why humans want to pass from that page to another. If it makes sense to link, don’t worry.
Be wary of footers, over-optimised anchor text and “unrelated” links
Spammy, manipulative footers, over-optimisation and random links on a page are a signal to search engines as well as your users. You don’t want to turn them off your brand.
Google “knows” when website are owned by a single entity. Don’t work too hard to “fool them”, just be authentic