A Completely Unofficial Rule: Cooperative Item Creation
As noted back in Part One, more than one character can cooperate in the creation of an item, with each participant providing one or more of the prerequisites. According to the rules, however, XP costs cannot be shared. One character must shoulder the XP burden alone.
If players in your game are avid magic item creators, you might want to experiment with shared XP costs. You can allow characters who work together on a magic item to divide up the XP cost any way they like. To share the cost, a character must provide at least one of the item's prerequisites. Any division of the XP cost is possible, provided that all the creators agree to the scheme.
If you have the kind of campaign in which some of your players pester the others to make magic items for them, you might want to allow any character to share the XP cost to make an item. An XP donor must be present each day during the item's creation (or at least when work begins on the item each day). Allow the XP donation to be strictly voluntary -- it doesn't work if the donor is magically charmed or compelled, or if the donor is bullied or intimidated into contributing. On the other hand, allowing evil spellcasters to force XP from unwilling victims might just add the right touch of nastiness to dark fantasy campaigns.