Send messages and reminders of their orders being shipped and delivery updates when it is in transit. More often than not customers forget that they placed orders and decline to accept it when it reaches them.
Optimizing your supply chain and providing speedy deliveries helps in reducing your RTO. Ideally look to fulfil the order within 3 days, orders that take longer than 3 days tend to be forgotten and hence are declined.
Ensure that products are packaged securely and compactly, which reduces the chances of damage during transit and thus reduces returns.
Offering customers a discount to pay online is a great way to ensure that they collect the order. Most products that return are COD (cash on delivery) orders. Offering a small discount to pay for their order is better than them paying for shipping and having inventory stuck in transit.