You might also assure them that you are not interested in harvesting anything from their tree wirhout their permission, although this is asking them to take a great leap of faith with a complete faceless stranger. Even though info about living persons is supposedly kept private, some people are still concerned about the possibility of identity theft.

It might be worthwhile to share any stories of success that you might have had in finding common ancestors with other matches and how you never could have done so without reviewing those matches' trees.

The problem is that there is no sure-fire,one size fits all method of obtaining access to every private tree. Even when you ask them to review your own tree, you are likely to receive a response from them like, "I looked at your tree and I didn't see any one familiar, so never mind." Of course you have no way of knowing how thoroughly they examined your tree, whether they looked for common geographic areas, etc.