“The surest way to stop writing about Rome is to stop.”
The deliberately obvious sentence mocks the difficulty of ending an overfull account when a subject offers more material than the writer can organize.
Source details
The Innocents Abroad (1869), Chapter XXVIII. Near the close of the long Rome discussion Twain says that the surest way to stop writing about Rome is to stop, immediately before abandoning his attempted guide-book chapter. Verified against the scan-backed 1869 first edition.