I like to imagine various “what would the protagonist do” type questions.
What music would he/she listen to?
What tv programmes would he/she watch?
Where would he/she go for fun?
What cereal would he/she eat for breakfast.
These things will probably never appear in the story, but they are just things you know.
I like to create little sketches in my mind of the character in different situations, and imagine how he/she would react.
Which then brings you onto thinking up the characters you need to support or bring conflict to your protagonist.
Probably the most fun part of the planning for me!
Some useful links: