insist on purchase order
2) only on receipt of purchase order should you supply the goods
3) terms and condition of payment should be specified in purchase order
4) further jurisdiction clause in case of disputes have to be incorporated in purchase order
5) there must be clause for payment of interest for delayed payments
6)if there is no purchase order even then you cna file summary suit under order XXXVII of CPC for recovery of dues
7) invoice and delivery challan constitute a contract and you can sue the purchaser on basis of bills raised by you . in such a case invoice should mention period within which payment is to be made , interest clause for delayed payment and jurisdiction clause