Rank on NoPayPOKER, at least, is based on the last 10 active days of play. So if you had a particularly good day 10 days ago then that profitability will drop off as you enter the next day. So your rank is based on your overall profitability not just the most recent days.
On other poker sites it may be different. The best way to build it is go slow and steady and play in cash games. Others may have different ways of building rank. Any comments?