Know how your client operates regarding POs and stay ahead of their requirements. Increasingly AP systems are tied to PO systems and despite having a signed contract you may have trouble being paid. Ask your clients about PO requirements:
are they necessary?
are they annual?
are they by project?
if you sign an amendment do you need a PO in an updated amount?
And remember- unless your contract says a payment term is dependent on PO provision, it's not a valid excuse for late payment (see last week's note on late fees...).