It costs a lot of points to raise attributes in GURPS. The cost is justified by the fact that attributes influence so many of the things your character can do. This page gives rules for raising an attribute in "installments".
At character generation, you can raise and lower your attributes at a cost of around 10 to 15 points per level; some very inexpensive lowerings give you only five points, i.e. 9-8 or 8-7, and some very expensive raisings can cost 20+ points, such as score of 16 and higher. After character generation, it costs double the point cost difference between the scores; this should immediately suggest to you that it is a good idea to spend points on raising attributes, rather than spending them on raising skills, because skills cost the same after character generation as they do before.
Because the cost of raising attributes after character generation is so high, it's a little unrewarding for a player to save up points and keep so many "in the bank" where they're not doing him any good. A "layaway plan" is presented here that allows players to spend points raising attributes in 5 stages and allows the players to realize some minor benefit for each payment. The player should keep track on his character sheet of how much he has paid in "installments".
For each of the four GURPS attributes, a table of five stages for each attribute is given below. The tables are presented in ascending order with the "least beneficial" items at the bottom, and "most beneficial" items at the top. (Note: I welcome feedback if anyone wants to contest what I see as being most/least beneficial.) The reason five stages are given (rather than 4 or 6) is because after generation, the cost of raising an attribute will be some number divisible by 5 (I suppose we could break each down into 10 stages, but that would be a little tough - I have trouble enough coming up with 5 entries for DX.)
Once the new attribute level is paid off completely, the full benefits of the attribute raising are realized, as per the normal rules, but incremental benefits are realized all along the way.
The alert reader will have already spotted a problem with this "layaway plan" approach: It is entirely possible that the only reason why a player would want to incrementally raise an attribute is so they can gain one of the lesser benefits. That way they only spend 5-15 points where they might otherwise have had to spend 20-30 and they get what they wanted for cheap! There are several ways to combat this abuse:
The Strict Approach - The GM mandates that every single point earned from adventuring must go toward paying for the new attribute until that attribute is paid off. This approach has the benefit of being the most simple and straightforward.
The Slightly Less Strict Approach - At the end of an adventure, the player must make as many payments as possible, but any remaining points (i.e. there's not enough for a whole payment) can be spent on skills or other stats.
The Much Less Strict Approach - At the end of an adventure, the player must make at least one payment toward the new attribute, provided he has earned enough points to make just one payment. Remaining points can be spent on skills or other stats.
One additional rule: The GM should not allow players to incrementally raise two attributes at a time. The "layaway plan" rules presented here represent special effort that a character is devoting toward raising a specific attribute; this attention can not be split up between two seperate attributes.
Here's a listing of stages for raising attributes. Each payment buys you one of these items, starting at #1 and ending at #5.
Disclaimer: The tables presented below are by no means set in stone, they are a work in progress (you will note that there are some items missing). I welcome suggestions regarding reordering these lists, adding/removing items from the lists, and any other ways to improve these rules.
Note: This is the list that I had the most trouble with and feel the least comfortable about. If anyone has some suggestions, please let me know what they are.