One tip from something I've tried is to drape the tarp over the ridgeline and then secure each end of the tarp to the ridgeline using 2 loops of cord and prusik knots. That way the tarp can be moved along the ridgeline and pulled as taught as you like without having to get out from under the tarp or re-tie anything. The knots seem to help prevent a bit of the moisture from running up the full length of the ridgeline too.