Alliteration is a device wherein a writer juxtaposes two or more words beginning with the same letter to draw emphasis or create a sense of cohesion. There are several examples of this in "If" by Rudyard Kipling, such as "treat those two imposters," "neither foes nor loving friends." We can also see consonance in "fill the unforgiving minute."
Assonance is similar to alliteration, except that it involves the repetition of a vowel sound, rather than the initial letter of a word. Assonance usually involves similar sounds in the middles of words. In this poem, we can see it in phrases like "fill the unforgiving minute" and "make thoughts your aim."
An example of symbolism in this poem might be "Triumph and Disaster," which are personified here to symbolize the "impostors" which a man might encounter during his life and be either dismayed or flattered by; here, they are represented as if they have human attributes, with each concept capitalized. We see the same technique in "the Will which says to them: 'Hold on!'" Here, Kipling's language and phrasing forces the reader to imagine a person's will as a living thing, which increases our understanding of its power.