Lilacs

They're in the title and in almost every section of the poem. So, yeah, of course we know that lilacs must symbolize something bigger than spring. From the very beginning we understand that those lilacs in the dooryard represent ideas of hope, perseverance, and the cycles of life and death. For the speaker though, they are also reminders of the passing of the one he loved (Lincoln).

• Lines 4-6: The lilacs are part of the "trinity" that the speaker uses throughout the poem. On a more personal note for the speaker, they also bring the "thought of him I loved" upon every returning spring. So, on a bigger scale they represent life's perseverance and on a smaller scale they will forever be reminders of Lincoln's passing.
• Lines 12-14: The image we get here symbolizes life's fertility and strength in that "tall-growing" lilac bush. But we're later reminded of its "delicate blossoms" that also appear to symbolize the fragility of life. So just in these lines alone, we see how those lilacs represent the full spectrum of life's strength and its delicate origins. 
• Lines 42-45: The lilac that the speaker lays upon a passing coffin is shared between each person as a reminder of hope and the enduring human spirit. Life continues and death can never fully "cover" the beauty of that unified spirit and the land we all share together.
• Lines 205-207: By the very end of the poem, the lilacs and the "trinity" that they are a part of come back as a reminder of the speaker's now-elevated understanding of death. The lilacs, star, and bird are all hanging out together in a sort of symbolic space that represents the speaker's newly-uplifted soul. Good times.