Update Your Resume - whether or not you think that you are at risk of losing your job, keeping your resume up to date is a good idea. You never know when you'll get a call from a headhunter or come across an opportunity that you want to respond to immediately. The idea is being ready to hit the ground running. Even though I feel very secure in my current position, my resume is up to date and ready to go.
Stay in Touch - its a cliche in job hunting circles that most people find a job by networking. Forget for a minute about formal networking - be sure you renew your contacts with old friends, colleagues and acquaintances. I have previously written that your colleagues and former colleagues are important career assets - so be sure to stay in touch or reconnect with some of your former colleagues who have moved on to other companies. This is not only good advice for prospective job hunters - it's also nice to reconnect with old friends.
To get started, make a list of people you need to reconnect with, and make it a point to give a call or shoot an e-mail to one person a day. This will also make transitioning to formal networking or job hunting mode much easier to do when the time comes, because your relationships will already be fresh and up to date.
Spread Your Name - being easy to find by prospective employers and headhunters is a good strategy in good times and in bad. Your action items here are limited only by your imagination - have a prominent profile on major social networking websites, give talks at industry events, write papers for your industry publications, write white papers or presentations that can be published on your company website etc. The important thing is to have your name attached to some impressive professional achievements and to make sure that these are easy to find online.
Make Discreet Inquiries - don't wait for the pink slip. If you think you are likely to be let go, start looking for a job preemptively. However, be sure to do so discreetly, because if your employer finds out that you are looking, it is very possible that you will find yourself on the lay-off list even if you were previously safe. In this context, I never felt secure about putting up my resume on the online job boards, since I am always concerned that my employer will somehow find out about this in spite of my precautions. Instead, I prefer more discreet job hunting avenues such as very specific headhunters, and job referrals through LinkedIn.
Emergency Fund - others have written extensively about the need to build an emergency cash fund for use in case of financial emergencies, so I will not belabor the point. However, if you suspect a lay-off is imminent, and you don't have a sufficient emergency stash built up, move quickly to correct the problem.
These are tough economic times, but luck favors the prepared mind, and much can be done to avert the worst consequences of a lay-off. The important thing is to actively engage the situation, rather than to wait passively for events to unfold. Make a plan, and start executing today.