I am totally in favor of securing written reference letters from those who can attest positively to you personally and professionally. Frankly, references can die, change jobs, stumble over their words, or simply forget. A written letter does none of that. I know you don't want to think of yourself as "forgettable" but it can happen. If you secure written recommendation letters, they serve not only as a reference, but as proof that you worked at said organization as per your resume.
Nurturing your references is less difficult when you've already secured their pitch for you via paper. If you have no written letters of recommendation, it's that much more important for you to nurture, or otherwise maintain quality contact with those who have agreed to serve as a reference. Send them a birthday note when you receive a notice of their upcoming birthday via your social networking account. Set up a LinkedIn account for professional use, find them and send them a note, forward them a helpful/relevant article etc. Providing them a reference on LinkedIn from your perspective as their employee/underling may impress them and encourage them to continue to speak highly of you. Send them a snail mail card during the holidays. I'm sure you can think of more ways - just act upon them because it's important! Don't wait until you need them to find out that you've let your connection with them lapse or you can't find them anymore. The more you keep up with your references, the more likely they are to give you their new contact info should they change jobs, numbers, etc.
The point is, don't let them forget you're a great employee and a wonderful person. Don't let them forget you. And remember, don't let them get lost! Try to secure a personal phone number/email address so you don't "lose them". How can you nurture your references if you can't find them anymore?
