This top quality bass soundpost setter is the tool for both inserting and adjusting your soundpost. Over 16" long, it is the preferred standard amongst luthiers. Simple and effective; you use the sharp point to "stab" the soundpost and place it into position, then use the other end to shove or pull the top and bottom into place.
I usually wrap a heavy string a couple times around the post, use the setter to place the bottom, and use the string to yank the top of the post into place. I can then release one end of the string and pull it out, using the opposite end of the setter to make the final adjustments.
Our current model has a rubber wrap around the middle section to make it easier to grip (and lessen the chance that you'll scratch the finish while fishing around inside the bass).
CAUTION: The tip is quite sharp - be careful handling this tool!