This probably depends on whose style manual you adhere to. Here's the way I was learnt:
When in first person (I/we), shall implies the mere intention to do or be something in the future. Will intensifies that intention. For example, 'I shall go to the store tomorrow' means a simple prediction, whereas 'I will go to the store tomorrow' implies a definite intention and commitment to do so.
When in second or third person (you/he/they), it is reversed. Will is a simple declaration of future action or being, and shall is a statement of definite intent, amounting to an imperative (command) on behalf of the speaker.
I believe the rule given above for the first person case has died due to common usage to the contrary. 'I shall go to the store' now sounds stilted in virtually every context in American English.
:-)