Holidays.
I had a Christmas type holiday in my campaign one time and the players spent a lot of jack for each other AND the party NPC's. Sure there were plenty of weapons and defensive rings and such, but they also got each other things like Bottomless Beer Mugs, a Pan of Frying, sparkly swords, things like that. They also hooked up a lot of non-party NPC's they'd been hanging with.
Birthdays would work well too. The DM would have to keep track of the calendar and try to work in the adventures during birthdays, especially the local Count's birthday (if they want to keep hunting monsters on his lawn).
There's all kinds of gods who have their own festival days. Probably a good idea to remember them before setting out on adventures. The Goddess of Love's holiday is a double whammy. You not only gotta hook up the goddess, but your special someone as well.
Last night our Monk/Cleric gave 1800 GP to a NPC fighting against to turn on one of our "arch-enemies", the arch nemesis had cast fly and was pelting us with magic missles from 150ft. up, the NPC proceeded to manyshot the BBE and drop him 150ft out of the air dead on the ground, good way to get players to spend gold lol.