Banks that want to increase their rate of cross-sell need to develop main bank relationships with more of their customers, according to new research.In its latest study of banking relationships, RFi found that main bank customers hold more products than non-main bank customers, have longer relationships with their banks and are more likely to consider the main bank when shopping for a new product.RFI Group managing director advisory, Alan Shields, said there were five key characteristics of a main bank relationship: it is the bank the customer transacts with most often; it has branches near the customer; it has ATMs near the customer; it is the bank the customer has been using the longest and it is the bank the customer uses to pay bills."Despite a steady decline in the use of branches and ATMs, these channels remain prominent in determining main bank status," Shields said.Fifty-one per cent of Australians report that they have changed their main bank relationship at some time (the global average is 41 per cent).An issue with a loan is most often the trigger for a switch, followed by customer service and convenience. "The credit card is often where the relationship starts to erode," Shields said.Life changes, such as getting married and getting a joint account, are also important factors.RFi found that customers in the 25 to 34 year age group had the highest propensity to switch, with 16 per cent saying they were very likely to change their main bank in the next 12 months.After that the propensity to switch diminishes steadily, reaching four per cent for people aged 65 and over.Shields said: "That spike in switching intentions is the key battleground for banks wanting to optimise their main bank opportunity."He said digital engagement was becoming an increasingly important part of that process. The average age of customers who are "highly digitally engaged" is 38 and customers in this category tend to give their banks higher net promoter scores.The dilemma is that the customers who are most engaged and most satisfied with their banks are also the ones who have the highest propensity to switch."They are happy with their bank but they are more demanding and keen to try alternatives," Shields said.Things that banks need to do to hold onto this group include making their mobile apps highly functional and easy to use, providing financial assistance and other forms of relationship management, and offering incentives to win business.