Bingo. There was a whole thread on this exact topic a couple of months back, and it took multiple pages to reach this point. It's just harder because you fight more gravity. Also bear in mind that you want to get as much air in as you can when you're suffering up a climb - easier to do that in an upright position rather than something more cramped/aero. Really though, it just comes down to doing it and working it out - I take it from your post that you don't climb much?