The income and expenses of a guild are sometimes not a focus of leadership, but they do represent an organisational factor that should be managed.
Cataclysm will help income significantly by allowing guilds to take a share of their members’ gold drops.
The usual sources of income I asked about in my poll, but it had too few votes to be a scientific result. Tankspot had a thread on selling runed orbs, whilst in earlier progression of course guilds distributed them using DKP.
Income can also come from actively trading and managing the donations from guildies, by judiciously disenchanting or auctioning those which are not taken out by other guildies.
Try to avoid being caught with a bank full of WotLK materials when Cataclysm is near. Sell everything before a release date is announced. If guildies want to use stuff for a new alt, they should put those into their personal banks.
There is some argument to keep WotLK gems or enchanting materials in a small supply in the guild bank, so you can gear-up your new greens at L82 and before L85 enchants are widely available, but my bias would be to sell them all anyhow.
Guilds don’t have many expenses any except paying for enchants for members, giving them valuable trade materials for new gear and repairs.
To sometimes offer gbank repairs to all appropriately ranked members during progression nights is a really good idea. It’s great for morale and helps people take the wipes with nonchalance.