Finish what you're working on before you leave to help (unless it's going to take hours, obviously). It's not too much to ask for someone to have a little patience when you're giving up some of your time. Also, don't give away so many items and/or so much cash that you go bankrupt.

Officers have some alternatives to doing wow powerleveling all ourselves. For instance, we can play matchmaker. If you know someone needs to do the same quest that someone else is requesting help with, recommend they get in touch with that person. It may be someone they've never really quested with, and it could be a great opportunity for members of two cliques to get to know each other.

Sometimes when I see a larger than normal number of people asking for help, I'll schedule a night where I offer my services to anyone who needs it. I'll announce it in advance and will not turn down anyone, of any level, for questing, rep grinding, recipe farming, running any dungeon, etc. The trick is to set a solid time frame, say two or three hours. Make it toward the end of a week. Then, when people ask throughout the week, tell them that you're busy now, but if they can wait till Friday night you'll help them with anything. That sets some clear boundaries between time that is yours and time that you're offering up to the guild.