Since I do most everything alone, I've worked out a few tricks for those types of tasks. Using ropes, cables, chains or more recently ratchet straps, I make a sling for both ends. Attach the sling by whatever means possible to a post or maybe even a simple temporary A frame or tripod. Raise one end of the board into the sling at nearly the required height. Repeat at the other end so the board is suspended by both slings. Then from a ladder or scaffold, it becomes much easier to to position the board into place and drive nails or screws. Sometimes only one fastener until you move to the other end and get it in place.
For heavier boards, I've done it more as a block and tackle type set-up where I can hoist the board instead of manual lifting.
It can be quite a bit of moving from one end to the other and back, but I find it safer than trying to lift and balance things.