One thing you want to be cautious about when determining the number of points to buy is figuring that the exact same vacation will cost the exact same number of points forever. Calculate a small "buffer" into the amount you buy.
While DVC can never change the overall number of points a room costs for an entire year, they can make adjustments to the
point charts that reallocate points from one season to another, change the dates of seasons, or change the balance between weeknight and weekend rates. The 2011 point charts were an example of these changes, when the point charts were changed somewhat dramatically. The difference between SUN-THR and FRI-SAT points shifted. It's entirely possible a similar shift is in the works for 2012. (In some cases last year, DVC maxed out the amount of change permitted under the contract. This leads many of us to speculate they'll continue with additional shifts next year.)
Some people were caught off guard by these changes. They'd calculated the number of points needed for their annual vacation to the point. Those who planned stays from Sunday to Thursday (to avoid the higher weekend rates) were the ones caught most off guard. While the cost of staying for a full week didn't change too much, the cost of these Sun-Thr trips really went up.
Banking and borrowing go a long way to shift the points you own to where you want to use them. These transactions can only go so far, though. Other options include renting points or paying for cash nights. The best plan, though, is to anticipate your real needs and get the points up front.
The other side of the coin, though, is the phenomenon referred to on the boards as addonitis. The more points you own, the more points you'll want.
We bought enough points for a week at SSR in a one bedroom. Then we realized we could spend two weeks each year if we stayed in a studio instead. Got ourselves used to two weeks each year. Then we started talking about how nice it would be to have the kitchen, tub, and laundry back. Need more points to stay two weeks in the one bedroom. Then we took some friends along for a trip. Realized how nice it is to travel with friends and family and decided to look into enough points for an occasional two bedroom stay. Then we stayed at the Beach Club for the first time and fell in love with it. Decided we'd better get some points there for the home resort priority.