A quick rule of thumb is to bank points prior to the 6th month of your use year if you know for sure you wont need any more for that use year. For example:
March use year:
If you choose not to take a trip at all during the use year, you would want to bank all of your points by the 6th month deadline, which would be August 31st. If you have used most of your points, but not all, you would want to be sure to bank them by the appropriate deadlines. If you have used all but 50% of your points, the remaining can be banked up to Novemeber 30. If you have used all but 25% of your points, they can be banked up to December 31st. No points can be banked the last 2 months of your use year.
Just remember that the number of points you bank into the next use year MUST be used up in that next use year, or they will expire. If you keep a running tally of how many points you bank, it makes it a bit easier to track. You might try getting Caskbill's planner to help with that.