By increasing the total value of Syndicates you place as a Captain. There are 3 Captain rebate tiers you can qualify for based on the total value of Syndicates you place in any given week:
Tier 1 (Bronze): Up to £99.99 qualifies for 5% rebate
Tier 2 (Silver): £100 - £749.99 qualifies for 7.5% rebate
Tier 3 (Gold): Over £750 qualifies for 10% rebate
At tier 3 (Gold), your 10% minimum Captain contribution to your Syndicates is effectively returned to you fully in the form of your 10% rebate. In other words, Gold Captains can play for free!