Why can't prevention of hypomanic episodes involve medication? The RIGHT medication? SSRIs taken alone, without a mood stabilizer, are notorious for causing (hypo)manic episodes. If you have been officially diagnosed with cyclothymia, you need to talk to a doctor right away about getting on the right medication. Lithium is the most famous medication for bipolar disorder, but it tends to work better in patients who are bipolar I and have classic, textbook "euphoric" mania. Bipolar IIs and cyclothymics tend to do better on one of the anti-convulsant mood stabilizers (i.e. Depakote and Tegretol) and/or one of the atypical anti-psychotics (i.e. Abilify, Geodon, Risperdal, Seroquel, Zyprexa).
Once you are properly medicated, then that is the time to watch for warning signs/symptoms of hypomania as others have suggested. You should keep a daily log of your moods and potential symptoms. You also need to try your best and set up a proper sleep schedule so you get at least a full night's sleep per night. Too little sleep or even something as seemingly innocuous as changing time zones/getting jet lag can cause a person to shift into (hypo)mania.
Also, I do have a question for you about your diagnosis- if you are not currently on medication and you really do have cyclothymia, you should be having consistent episodes of hypomania every 2-3 days, followed by dysthymia (low-level depression) the next 2-3 days. The DSM criteria specifically states that you must have continuous alternating episodes of hypomania and dysthymia for at least two months. If you are not experiencing this, then you should seek a second opinion about your diagnosis. If you're having full-blown depressions and hypomania, with periods of normal (euthymic) mood in between, then you most likely are bipolar II.
Feel free to ask me any questions.
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Helinger: Now, what do you see, John?
Nash: Recognition...
Helinger: Well, try seeing accomplishment!
Nash: Is there a difference?