Happiness makes up in height for what it lacks in length.
Source: A Witness Tree, Robert Frost, 1942
This is not merely a line by Frost — it is the title of his poem 'Happiness Makes Up in Height for What It Lacks in Length,' collected in A Witness Tree (1942), the volume that won him his fourth Pulitzer. Attribution is secure and the source is a published primary text.
The best way out is always through.
Source: North of Boston, Robert Frost, 1914 (poem: 'A Servant to Servants')
From the dramatic monologue 'A Servant to Servants' in North of Boston (1914), where the speaker reports her husband's saying: 'He says the best way out is always through.' Worth noting for corpus honesty that in context the line is a character quoting another character, not Frost speaking in his own voice — but the wording and the source are secure.