Having lived through the dot come bust and other economic failures I have found that making sure you stay valuable and relevant are the most important factors. Someone pointed out making sure you are valuable to your current employer by taking on the parts of code that everyone dreads. That is good, so is just making sure you are a team player and are pulling your weight and pitching in wherever you can to make sure things stay on track. In the past, I jumped on a support desk, or stayed late to stuff mailings with sales staff etc. When people see you are willing to help make the team successful even outside your defined role and that you are showing loyalty most good people will return the favor. BTW -- all the things I did when times were less then ideal was what made being an entrepreneur easier, because sometimes you just have to suck it up and clean the bathroom before a client visit, or after depending on the situation.
If you have interest in starting your own thing, there is nothing wrong with doing so. Minimize current expenses, pay off as much of your debt possible while you are a FTE and save some cash up. While you are still working start your side business and take some time to get it right. Starting a business in a downturn is many times quite successful because many businesses look to outsource projects, minimize new head count etc.
One last point, many times enterprises use downturns in the startup sector to snap up talent that they have been squeezed out of getting. So usually enterprise hiring remains pretty robust for the first 12-18 months that I have seen. And this time around if there is a bubble in the startup side, enterprises are sitting on some of the largest cash reserves that I can remember seeing, so who knows how that will work out.