The trick is to start very slow, and stop before you're in pain, because you will start to feel it after you stop.
If your pain is good, either increase it by 30 seconds or walk a teensy bit faster.
This is how I got myself out of a flare up so bad I barely left my couch for nearly 6 months. As for motivation, I find that the slow start helps. Is there a time of the day you feel better? I'd do it then.
Weights are also good, the same rules apply. Start very very slow (and light weights) stop before it hurts, and listen to your pain afterward.
The trick is a little bit of exercise frequently.