How to reduce NET and CPU usage in Private EOS

Easy: Use eosio.bios instead of eosio.system contract

Advanced: newaccount in eosio.system initializes account's resources with 0.

   void native::newaccount( const name&       creator,
                            const name&       newact,
                            ignore<authority> owner,
                            ignore<authority> active ) {
      /* ... */

      user_resources_table  userres( get_self(), newact.value );

      userres.emplace( newact, [&]( auto& res ) {
        res.owner = newact;
        res.net_weight = asset( 0, system_contract::get_core_symbol() );
        res.cpu_weight = asset( 0, system_contract::get_core_symbol() );
      });

      // three zero params mean ram, net and cpu in order
      // you can change these to proper values, or set -1 to make them unlimited
      set_resource_limits( newact, 0, 0, 0 );
   }

You can't just reduce the resources usage if you want to get an alternative and don't have EOS tokens for stake see the REX(Resources Exchange System) in eosio documentation.

Also must watch this video for simple explanation of REX video.

For explanation of different REX functionalities see this blog article