What works for me is to anticipate the steepness and length of the hill and drop down to a low enough gear that I know I can make it 100% of the time. I will get in the rear cogs needed and then when I get to the spot on the hill where I am slowing down I go to the inside chain ring for a very low gear. No worries about a missed shift with this approach. I am not in a race so coasting into a hill is not a concern.

If I am in too low a gear it only means take 1-2 minutes longer to get to the top. No reason to take a chance with too high a gear and having to change gears or get off the bike and walk.