One of the techniques used by Frost is repetition. Successive sentences beginning with "and", as demonstrated in the first stanza, saying, "AND sorry I could not travel both/ AND be one traveller.../ And looked down one as far..."
This is indicative of Frost's persona's extended deliberating over his decision.
Frost changes from past tense to future tense in the last stanza, thus developing an anticipating, predictive tone, showing that he assumes he will be happy of his choice - the road less travelled - when he looks back on it later in life.
The poem is written in first person, giving it a very personal tone, and allowing the reader to observe the workings of the persona's mind
Inverted word syntax - again, demonstrates deliberation and decision making, shows reader what he is thinking
extended metaphor throughout the poem, representing different journeys/paths we take on our inner and external journeys.
this is really helpful, thanks!