I stopped last summer to help someone one a roadbike who was walking. He had a flat, so I offered to help.I broke two normal plastic levers trying to get that tyre off. He did say that the bike was new (so the tyres probbly new and stiff too). I then rummaged around in my bags to find my favourite tyre lever: A single-handed thingy I just couldn't find at first.

It is this one - crankbrothers Speedier Lever:




I actually have two of them now. I thought I lost it somewhere, so I panicked and ordered one immediately, only to find the old one the very next morning.

Also, I may be telling you how to suck eggs, but with tight tyres, make sure the edge you do have over the rim is situated in the "bowl" of the rim. It gives you much more room to work the rest of the tyre over the rim. Of course, here it really helps having a one-handed lever.