old hard tyres can be hard to get off sometimes,
but what most people forget to do when they are trying to get the first side off i, to eveny push the tyres sidewall and bead down into the deeper recess, centre part of the rim.
this way the tyres' bead has a heap of room to move as the outer edges of the rim are shallow, the centre section is deeper.
i normally kneel on the sidewall of the tyre at about 4 o'clock and 8 o'clock.
then put the tyre levers in at 12 o'clock and start from there.
try to keep the tyres' bead in the centre the whole way around.
i think you'll find this is the easiest way to remove a tyre.
when you go to refit a tyre do it basically the same, once you have the bead over the rim, try and make it sit in the recessed centre section, so it doesnt stretch the tyre as much.
and yeah a bit of talcum powder will help the the tyres slip over the rims, i have never used it though.
when you have the tyre off, take the rubber rim tape off the centre part.
use a bit of rubber grease on each spokes nipples too, in the threaded centre, and around the outside of it.
this way you will be less likely to get water into the inside of the rim and rust up the spokes' adjusting nipples.
then when you go to unbuckle your rim or tighten your spokes, it wont snap the spoke because it has rusted solid to the nipple.
refit the rim tape too, before you go and fit new tyres or tubes.
cheers, craig