Depends a lot on soil conditions and whether you are likely to
encounter many plant roots. Another factor is wind exposure - you
might need deep holes and concrete.

If soil conditions are tough, I found a drainage shovel (http://
http://www.abbeypowertools.co.uk/bui...182-64331.aspx)
indispensable for dislodging difficult bits - the levering action also
tends to compact the sides of the hole as you attack the bottom,
resulting in clean, deep, solid holes.