I recommend being liberal with adding people you meet on LinkedIn. That's not to say you should be spammy or add people you don't know, but if you meet someone at a conference tell them you'll look for them on LinkedIn.
Once you've grown your network of contacts, be sure to update regularly and with useful content! But not TOO regularly.
Also bear in mind that with LinkedIn, you are catering to working professionals. I prefer to update my profile early on weekdays, because I suspect that'll get my changes to the most eyeballs.