Writing retreats, like writing conferences, are con jobs. If you want to write, then you should write. And here’s how you can retreat yourself:
a. Find an isolated place – hopefully a key setting in your book – and go there. 
c. Arrive and unwind. Don’t worry about writing on the first day. 
e. Never get too down (or up) on your work. Just keep writing. A few words is enough.
f. Be active. You have to get out and circulate your fluids.

So no computers,.. you had the Olivetti out!
No on-line connections. I stand corrected. Although an Olivetti is a damn fine idea.