There are many ways to generate leads, which you can read ad infinitum elsewhere on these boards.
However, regarding the specific SOI question...
DO NOT go into this field expecting business from your friends. DO, however, go into the field expecting to get business from your friends' friends.
I highly recommend that you get and read Jennifer Allan's books (see
http://www.sellwithsoul.com/ for info); she explains quite well how best to work your SOI, and certainly much better than I ever could. Yes, you want people to remember that you're in real estate, but you absolutely do not want to be pushy about it. Her general theory is that by simply being a friendly person who stays in regular contact, there are plenty of opportunities to talk about how things are in real estate.
Whatever you do, don't send out mass "I really love your referrals!" types of emails. You'll only alienate the people who should be helping you. And you really don't need to do that at all. I've found that people tend to enjoy talking about real estate, even if they aren't planning to buy or sell themselves. It's those casual conversations that will come to mind when one of their friends, neighbors, or family members is thinking about buying or selling.
The other thing I recommend doing is to EXPAND YOUR SOI as much as possible. Simply put, just get out and meet people. Get back into that club whose meetings you stopped attending. Meet up with your former co-workers for happy hour. Accept every dinner party invitation you possibly can. Last weekend I got two new leads simply by attending a dinner gathering, and now we're Facebook friends (on my personal, non-business page). They aren't going to be buying/selling for some time, but when they're ready, I'm willing to bet that my spending a few hours chatting with and getting to know them are going to be a better guarantee of their business than any number of leaflets, postcards, or door hangers they get in the meantime ever will. Plus it was fun anyway.
Keep in mind that the SOI technique is not always quick, so other lead generating methods are often necessary until you have a well established referral network. Once you've got some experience under your belt and can demonstrate that you're truly successful in your new field, you might be able to convert a few of those friends who would never work with you into clients as well.