Janet Baraclough wrote:
[...]
In deep loam, a post with a point tip can be rammed in with a
sledgehammer (use a protector on the top surface of the post to
avoid
splits). In stony compacted ground, where a hand augur would be
useless anyway, use a pinch-bar (to loosen soil) and narrow blade
spade (to get it out). Deepest sections are done lying on the
ground,
whole arm down hole, wielding a catfood tin or similar :-)
[...]

I've always used a crowbar to make a pilot hole so the post stays
straight when driven in with the sledge-hammer. There's a kind of
post driver which looks very good for those who aren't used to the
big hammer, but I don't know what it's called: a sort of sleeve goes
over the top of the post, and you slide a ram thing up and down using
both hands. Very safe, too: I once recommended it to a blind
conservation group. It may be in hire shops.

Mike.