There are many methods for people to manage their emotions, ranging from mental training to medication. In all situations, a qualified, trained mental-health worker (such as a psychiatrist) should be consulted to evaluate a person's mental state before any action is taken.
One very useful method of controlling emotions is meditation; the success rate varies from person to person, but in general the relaxation and self-introspection allowed by meditation is useful in keeping a person's emotions centered. Some, especially those who dwell on past events, might have negative reactions to meditation; to gain benefits, one should seek out a teacher (not necessarily medically-certified -- this is both a pro and a con) to instruct in the desired meditation skills.
Another non-medication tool is public meetings such as AA or other support groups. Many people find that speaking their mind in a non-judgemental space is useful in managing their emotional status; by letting all their feelings show in public, they no longer have the need to "bleed off" emotions through private response, self-harm, or violence.
Many psychiatric patients have success managing their emotional state with one or more prescribed medications. While non-prescription remedies such as St. John's Wort can be useful (especially as a placebo) all medical questions should be referred to a qualified expert. Many people have unexpected reactions to medication, especially if prescribed two or more, so it will take time to fully understand personal reaction. Always consult a doctor before taking medication for any reason.