I go outside (you can do it with a friend, a young kid is always good) and play the Story Game...It is where you look at something or someone and make up a story about them. You take also take picture and build a story from it.
The real answer is to write even when you don't have motivation.
Of course, that is much harder to do than say, but that is the real answer.
I've heard somewhere that WillPower doesn't really exist, so motivation might also not exist.
Some people give over their entire lives to writing. Thomas Mann wouldn't even interrupt his writing to attend the funeral of his son, who had killed himself. Genet was forced to write on toilet paper, as that was all he had during his years in prison. When the guards found and destroyed his life's work, he began again, recreating what he'd done from memory. Dostoevsky spent many years in a prison camp in Siberia, where he wasn't allowed to read anything but the Bible and was given no writing materials - just hard labour. But he continued to write when he got out, despite the fact that Russian law prohibited a former prisoner to be published. When the tsar read Dostoevsky's House of the Dead - given to him by friends - he cried, lifted the ban, and allowed the work to be published. Conrad, a Polish refugee, taught himself English while working on a ship, despite the fact that he didn't speak a word of it until he was twenty years old. Through sheer devotion, he turned himself not only into a proficient writer but into one of the great masters of the English language. Faulkner laboured in factories and post offices while he write his works. He said the great thing about being published was that he was 'no longer at the mercy of every bastard who had five cents for a stamp.'