Use a multinetwork IM program. There are competing IM networks for users, with AOL Instant MessengerSM, MSN Messenger, and Yahoo! Instant Messenger the leaders. A user of one network can't communicate with someone on another network. Trillian for Windows (www.trillian.cc) and Gaim (http://gaim.sourceforge.net) for other operating systems let you operate on multiple networks simultaneously.
Send descriptive links instead of URLs. Most IM programs have an easy way to create a hyperlink. Instead of pasting a long URL into the conversation, create a link describing the content of the page you are recommending (e.g., "Demographic info of Berlin" or "Link to book review").
Employ away messages. It's poor service not to explain why there is no response.
Create a profile. It's a great way to convey information about the library and increase your online presence.
Accept imperfection. Notice a typo 30 characters back? Don't correct it. Most words are easily recognizable through context, and typos are accepted—if not expected—in this medium.
Use abbreviations. Most online users save keystrokes with acronyms. Get used to it. Many people know that "LOL" means "laugh out loud" but what about "FWIW"? Google it. Of course, YMMV.
Never panic. Speed is important, but don't feel rushed.
Try to use only online sources. But don't be afraid to tell people they'll need to come to the library. Also, be willing to scan a print document to PDF and send via email.
Load IM software on public PCs. Let users online in the library get help without having to go to the reference desk.