Although the Internet has been a boon to networking over the past decade or so, there is something to be said for a face-to-face networking over business lunches or trade shows. Adding a personal touch that might be missing from cold emails and Twitter, a lunch with a colleague can be an invaluable tool for you to use for everything from job leads to getting advice about a venture or idea. Trade shows offer the chance to speak with peers in your field. They know that you are interested in finding a place of employment.
People are more apt to share more personal details, and are definitely better able to read you as a person when sitting across from you at a business lunch, or spending a little time with you at a trade show. The chance to understand a person's inflections, and gauge their sense of humor (or lack thereof), their personal foibles and idiosyncrasies or how much you might even want to engage with them outside the business milieu can only be had in a face-to-face encounter.
There are certain protocols involved when you are considering networking through business lunches or trade shows. Some are common courtesy: such as not interrupting a vendor while he's currently dealing with a peer or customer. Wait for your opportunity to introduce yourself with a firm handshake and a business card, and get to know the person a little bit before you dive right into a lunch invitation. You might find that after a little personal interaction that you are no longer interested in getting to know someone better, or it might cement in your mind that this is someone to get on your side. Be interested in their products or sales pitch (it's a hard, sometimes thankless job working a trade show), and ask pertinent questions.
Know that once you've proffered a lunch invitation to a potential networking connection, he or she may not want to talk business. Trade shows often carry with them grueling schedules, and lunch may be one of your new contact's only break in the day.
You might consider making initial contact through the trade show venue, then following up with a lunch invitation. Be sure to let your contact know the agenda of lunch; don't lull them into thinking a personal relationship or friendship is on the table when you're really looking for a networking or job opportunity. Be sure to propose a firm date and time, with the offer of alternatives available. When you are networking through business lunches or trade shows do not invite anyone other than the main person (i.e., don't show up with coworkers, or your girlfriend). Be prompt, be appropriately dressed, be courteous, and thank your invitee for their time. Once you have had lunch, remember, it is up to you to pick up the check, even if they insist. You certainly wouldn't be impressed if you were invited to lunch and were expected to either pay or go halves; neither will they.