I've always seen guilds as somewhere along the lines of a small company, a martial arts school/group, an order of mages, a gang, or a fraternity. In that case, belonging to more than one depends on what types of guilds they are. You could be part of a glass-blower's guild and a member of the Klyros Hidden Wing fight school. You wouldn't be able to be as dedicated as if you were only a member of one, but you could still do it.
On the other hand, if you're a member of the East Hydlaa Lurkers gang, chances are you won't be a member of the Ojaveda Ulbers gang too.
Also, some are something that you're generally included in even when you "leave" and move on in life. Kinda like the military. The fraternity style guild would be like that. Martial arts style guilds could also work that way, unless you actually leave the style, and use a different, possibly rival, martial arts.
I like the items idea. Besides the OOC part of a guild banner, it just doesn't look good. Plus, it makes secret organizations more visible. In reality, you don't know somebody is a member of something unless you see some badge or uniform, or use the trademark move or spell.