Right, well, I've learned a couple of things from this site in the last couple of days:

Buffing - I'd never heard of buffing, but I checked out the 'pass' guide on wiki and this is solving my upper lip problems above the sides of my mouth. You have to remember to flatten the angle of the blade against your face, though.

Also, I tried this (below) technique Rayman talked about which involves using the muscles in your top lip (or bottom) to draw it down in a controlled way against a stationary razor.
http://straightrazorpalace.com/shaving-straight-razor/46507-how-shave-under-your-nose.html

Without doubt, straight-razor shaving is the least straight forward thing which I thought was pretty straight forward, I've ever done!

All the best,
AD