A little light exercise can help with the weight gain issue whilst also being positive for your mental and physical health. It doesn't have to be intensive exercise - just add in an extra half an hour of fast walking a day. Also try to avoid over eating. You may not be consciously doing it but the most common reason for weight gain on these meds is that they increase appetite which leads to consuming more calories. Keep your hands busy and mind distracted when hunger strikes.

You can also fill up on veg (frozen mixed vegetables are great, easy to cook and still healthy).