We talk more about breaking down silos than preventing them from sprouting up in the first place.
We've met the enemy and they are us. We create the conditions for silos in numerous ways, including:
Paying too much attention to the org chart
Not thinking about processes from start to finish
Telling people to work together and nothing else
Silos come in so many shapes and sizes. Ignore at your peril and they'll unravel the best of intentions.