The day of the month of which you bought your Premium Membership is your "tier date". All tier will be collected from you on that date.
On the tier date, you are charged tier based on:
a) the largest amount of land you owned during the period since the last tier date, PLUS
b) _all_ land credit you have contributed to groups (even if the group is not using using that tier to own land.)
On a), beware that it doesn't matter if you only owned a certain amount of land for 5 milliseconds - it will still be counted as your "largest amount of land", and you will be billed based on it. However, it's only based on your highest ownership at one moment. If, during a month, you buy a 512, sell it, buy another, sell it, buy another, sell it, then buy one more, you will still only be at the 512 tier (ie, no extra "tax"
because you did not own more than 512sqm of land at any one point during the month (even though 2048sqm of land has been owned by you at some point during the month).
On the Second Life web site, you can adjust your "tier level". This is the maximum amount of tier you are willing to pay. Whenever you take an action that would require you to pay more tier than your tier level, you will be given a warning box and required to either increase your tier level, or abandon the action. However, if you take an action that leaves you paying _less_ tier than your tier level, then your tier level does _not_ automatically come down, although the amount of tier you are actually charged _does_.
Here's an example: as a Premium member, you try to buy 1024sqm of land. You will get a tier level warning, and you will have to authorise the $5 tier level (for the extra 512sqm extra). Near the end of the month, you sell 512sqm of the land you bought. You will still be billed $5 for the current month (because even though you sold the extra 512sqm, it's still part of your historical maximum usage for the month), but then in future months you will not be charged tier as you own only 512. However, if you decide to buy another 512, you will _not_ be asked to authorise the $5 tier level because you already authorised it back when you bought that first 1024. Your bill will just automatically increase. Note that you can manually lower your tier level if you want to be certain of knowing whenever your bill increases. (The main reason for the tier level system is so that residents who buy and sell a lot of land don't have to constantly deal with their tier level bouncing up and down - they can just authorise the maximum level once and then buy and sell as much as they like.)
