With banking and borrowing we do one big trip every 3 years or so. Here's how it works. In my case we have 375 points:
2009 points: we will be banking 31 points into 2010. These are points we will not be using in 2009 and if we don't bank them, we will loose them. We must use these 31 points in our 2010 use year.
2010: We will get 375 points, bringing us to 406 points.
2010: We will be doing a big family vacation, using 722 points (BWV, standard view, 10 nights, 1 2BR and 2 studios). We only have 406 points available, so we will borrow 316 points from our 2011 use year. Note that the 316 borrowed points must be used in 2010, and if we don't use them in 2010, we would then loose them.
Before deciding how many points you should buy, you need to figure out your probable vacation plans. The suggestion of buying smaller contracts is a good idea. Our points are broken into 2 separate contracts. Also you should consider buying resale since it is a lot cheaper. The sponsor of these boards whose link is at the top of these pages is one of the popular brokers. We used the sponsor and were quite happy with them.
Good Luck!