Hi Andy,
Would you consider an early settlement reward? It could be that you
charge X to settle before the due date and your bill is X + Y
afterwards. With Y getting bigger the longer they don't pay. That
could lead to more invoicing work 'though.
Or give (say) 2 additional days hosting free (at end of term) for
early settlement - that way it doesn't cost you physical money, would
probably be easier for accounting purposes, and won't mess with your
headline prices.
I think sending out the invoice one month before, then spamming with
weekly reminder emails is a good approach. And daily in your final
week before payment is due.
Thanks,
Duggie