There are a variety of different outside forces that might trigger a psoriasis inflammation.

• Alcohol.  I prefer not to acknowledge this possibility because I really enjoy drinking beer.
• Smoking tobacco.  Smoking is unhealthy anyway.
• Excessively high stress levels.
• Heavy caffeine consumption.  I have experimented with this one and have found no conclusive evidence, since my Psoriasis comes and goes during long periods of time during which I am consuming caffeine.  At the same time, when I stopped consuming caffeine for a while, none of my Psoriasis symptoms improved.
• Cold weather.  I have found that a combination of cold and dry weather is especially bad.
• Too much sun (a sunburn) or not enough sun.
• Certain drugs.   You should consult your doctor to confirm whether or not any drug you are taking might be irritating your Psoriasis.

I have also heard that basic foods such as eggs, dairy products, citrus and red meat can cause psoriasis inflammations.  Honestly, I am a bit skeptical of this list.

As a general rule, anything that alters your body might cause a psoriasis inflammation.   On a related point, the more healthy your body is, the better equipped it will be to fight off any disease (including psoriasis).