Video calls. If you aren't having at least one video call a day something is probably wrong. Configure it such that starting a video call takes no more than 4 clicks.
Have a company-wide General/Coffee chat where people talk about arbitrary things. It's better if this chat has history which expires in 24 hours.
Write lots of short documents -- especially for designs. Review them much like you would review code. This can be as simple as Markdown documents in your repository using your normal code review tool. Ensure all documents are listed in a single easy-to-find index of some sort.
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