Use a unique order number for each transaction attempt

Required – NO, but you should send unique order number to CCAvenue. If it is not unique CCAvenue will not Flag, but you will have duplicate entry in CCAvenue sales report.

Store your unique order number for each transaction attempt

Before sending a transaction to the Payment Server, you should store this unique order number with the order details in your database. The merchant order id is returned in the Transaction Response along with the CCAvenue system generated unique transaction reference id.

The unique order number can be used for the Order status function to retrieve the transaction status of an order that was lost or missing.

Store your access code and secret key securely

Make sure your store your access code and secret key securely.

It is recommended to store your Secure Hash Secret in a secured database, or in a file that is not directly accessible by your web server and has suitable system security permissions.

Change security has regularly..

If you are using the seamless integration, get PCI DSS certified

In seamless integration you are saving or taking customer payment cards information at your end then you should first get your website PCI DSS certified.  Suggested not to go with seamless integration.