On paper. Made (reasonably) permanent.
You put something in writing when you want to give them assurance of authenticity: verbal agreements aren't worth the paper they're printed on.
You ask for it in writing when you don't trust them to follow through on their promises.
You ask for it in writing when you know they will forget what they have said.
You ask for it in writing when you want there to be a record of what has happened: because sometimes a paper trail is important.
Get it in writing. Be sure.
This message brought to you by the Paper Council.