Restrictive clause

You are having a good business, engage a local lawyer and proceed as advised.

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If its your own design you can patent it which will give lots of hold on your design,

Non disclousure agreement is good because it binds the party not to disclose to the third party any facts coming in knowledge to him during course of his business 

Register with every costumer Non disclousre agreement as a clause in the main agreement then you have option to recover for damages caused to you,  for drafting take help of a local lawyer

No query.  Where is the query?  The message seems to have been deleted by the querist, hence no reply offered.