Your profile doesn't have to be long or complex, but it should include your basic information - current and recent employers, industry, interests/hobbies, a brief bio, and links to your websites or anything else you've published online - blogs, articles, etc. Also, be sure to have your photo handy, both online somewhere and on your hard drive, and in a variety of sizes, so you can make it appropriate to the site's profile layout.
Whatever you do, do not set up an essentially empty profile. Most sites will let you poke around as a guest without setting up a profile first, and an empty profile isn't helping you, so there's nothing gained by doing it. On the other hand, most sites have some way of showcasing new members, so if you set up an empty profile, you're missing out completely on the additional exposure you get as a new member, not to mention whatever damage you may do to your reputation. If you've already done this, don't sweat it too much - people are also pretty forgiving. Just get in there and get it done.
