Raised Bed Liners
Raised beds will dry out fairly quickly which is why we recommend a liner to help retain the moisture. If you are on clay, this would certainly also help prevent your raised bed drying out.
The liner will also help protect the wood in your raised bed from constant contact with damp soil, helping to extend its life further still. I always line raised beds for this reason.
I hope this helps. Please let me know if you require any further information.