Best treatment for lifelong lsd flashbacks?

Unknown. Since the cause of flashbacks is mostly unknown, i.e., could be related to situational memory states, simply a memory cued like others with a sensory trigger (music, light, smell), (unlikely) sequestered drug being released, some other neurological cue (trauma-like reaction) or process, the treatment is not widely known if it exists. Regular primary care, healthy lifestyle &drug abstinence is key.