Keep an old toothbrush to hand with your window cleaning cloths and use it to brush the corners of the frames where dust and grime gets stuck.

Get an old rag and pit it tight around the tip of your finger and pit your finger in your window and clean it. Or get a Q-tip.

all of those but in my room i use a mascara brush to sweep the dirt away and then dip an old toothbrush in the cleaning liquid that im using. it works!

get a crochet needle and put a cloth over one end. soak it in water and then and juck the end of it into the corner. trust me it works.