Think of /Think about
This tricky pair is more nuanced. One choice suggests a specific choice and the other suggests pon- dering over something for a while.
He was thinking of a number between one and ten. (specific choice)
He was thinking about going to law school. (pondering)
Here is my trick to remember which is which for this pair: Of is short, like making a quick decision. About is longer, like pondering options.