I don't have one as big as BHW. I run a couple small paid membership forum/communities. I have one newer one with just over 500 members paying $70/mo. So I don't have the 1000 yet but will within the next 2-3 months.
Here are 3 quick tips with private forums...
1) balance of good content - not lots of content. Too much will overwhelm users and they'll leave. Too little/not high quality will make them leave.
2) Salting the mine! until you reach over 50 members, it'll be quiet. You'll need to create the conversation and you'll need to have a few "ringer" accounts that make it look more active. After you reach a good number, this'll be good on its own.
3) Limit number of topics. It is tempting to want to have 10+ topics. Don't! Stick to no more than 3 sections at the start. Any more than that, your forum will look like a wasteland! Expand topics after it makes sense.
As for promotion. I have a good list and do content marketing. Nothing magic.
I will say that in my promotion, I close and open enrollment. It is not just open all the time. Every few weeks, I open it up for 3 days and close it. Hope that helps a little...
Cheers! And Good Luck!