How can you stop self harm before it starts?

Be in therapy: If you have ongoing impulses to harm yourself -- either suicidal or other things like self-cutting, etc -- you need to see your physician. S/he may refer you to to a psychiatrist or therapist. Psychotherapy can help you understand and deal with what's happening inside that triggers these impulses. Although it can take time and patience, this work can eventually "stop self-harm before it starts.".