The weird thing about writing is that its almost like being possessed by some weird spirit of dreams and thought. Huge triggers are music, films, life experience, intense discussions, a picture, in a book or something fleeting but somehow persistent in the mind. Its hard to do on command but can be directed when you know what is the mood to go towards and what is the idea to be traced. Its good to have a plan outline to understand rough direction but it can happen that when mid writing it just refuses to play out as originally thought of. Its weird when that happens but instead of battering on and refusing to change concepts its good to take a step back and think of other possibilities. If it wants to go in a certain direction then it will flow with a little inspiration. Then its good to go over it and make sure the concept is logically connected.