Research at workshops by the association suggests firms are underestimating the extent of lost clients.
Some 62 companies representing the finance and insurance sector participated in the survey.
Only 36 per cent of firms felt there was clear guidance from industry bodies when dealing with 'gone away' customers. This fell to 15 per cent when dealing with unclaimed assets.
Some 84 per cent of companies said they made an effort to trace lost customers but 38 per cent were unaware of how much they were spending on tracing customers.
The guidelines will ensure customers' data is accurate, regularly maintained, adequately protected and effectively linked in providers' IT systems.
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Carol Knight, TISA director of member services, said: "This is the only way we are likely to see a significant reduction in the number of dormant or unclaimed accounts and at the same time increase investor participation.
"We have met with the FCA to share progress on this project. The FCA is currently reviewing the Client Assets rules and has agreed to continue engagement with us as we develop the proposed industry standards."
The new best practice guidelines could qualify for a kite-mark that firms could use on their customer-facing material.

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