Chronic pain and illness can be quite difficult for people, and one of the best techniques to help keep the mind from becoming negative is the D/D technique. That is, understanding that any pain that we have, there are others who have worse pain, so that we don't fall into self-pity. Falling into self-pity is very detrimental for the mind because then what we have to do seems bigger, so protecting the mind from falling into self-pity allows us to deal with the pain that is present.
Sometimes fear can arise when we have pain, and we project it into the future and increase it in our mind. Learning how to deal with the pain that is present and not projecting it is very important, because dealing with one moment of pain after another is a lot easier than dealing with fear of twenty years of projected more pain. It's beneficial to learn how to come into the present moment and see when pain is actually present and when it is not. There's usually, with all sorts of pain, some moments of fading away or lessening of the degrees of it.
Another thing is to lessen the idea of the body being oneself, so we don't take it personally. Sometimes there is a tendency when a disease or illness or pain arises for unworthiness to arise, "Why did this have to happen to me?" "What did I do wrong?" We may take it personally because our body is not as good as somebody else's body. So seeing the non-selfness nature of the body, how pain just arises due to causes and conditions is very helpful so that we don't take it personally.
As far as pain and illness, we also have to understand our limitations, sometimes we will try to do more than is possible. This ties in with not taking it so personally, and understanding our limitations and working within those limitations.
I'm also a strong believer in learning how to understand the true causes of illness and seeking ways to lessen it through our own efforts. So another way to deal with it is to research it and try to see some of the causes of it and lessen the causes of it by appropriate exercises, food and appropriate medicine. As well, learning how to do the Unpleasant Physical Sensations technique is very important, so that our pain threshold grows, we're able to deal with increasing intensities of pain with less reaction in the body and mind.
One other way to deal with chronic pain is to learn how to have rest from it. That means knowing how to do types of concentration practice in meditation so that we don't feel the body so much. Meditating more on confidence-inspiring objects. One of my favorites is reflecting on the Buddha. This can give us a feeling of courage and take our attention away from the body to a very uplifting object. Any type of joyful meditations, such as Sympathetic Joy, reflection on one's good past Kamma, especially if you've done a lot of good past Kamma, is very helpful. It can not only lift the spirits but help us to rest from the pain. Other people may go with the breathing, but I find that the breathing tends to be too involved with the body when I've got pain, so sometimes a conceptual type of meditation helps to get one's attention away from the pain and then that one can have periods of rest from it. Being constantly with pain is very tiring for the mind, so using reflection meditations like Compassion/Lovingkindness and the others I've already mentioned are helpful. We have to be careful with our subject though, as sometimes if we're thinking about Dukkha too much it can be difficult when we have pain, so I prefer the confidence-inspiring objects, like Sympathetic Joy.