Your clearance is good for 24 months once you stop needing it irregardless of the level. Once it expires, it's like starting over, no grace no nuthin'. Companies are very selective in who they hire without clearance because:
1) the process takes so long and they have to pay you to do nothing while you wait in the tank
2) the process normally takes from 4 to 24 months and they have to kickin for some of the cost
3) you can't be billed out during that time b/c they don't have uncleared work
It's like super negative equity for them when the can hire someone else right away who adds to the bottom line. Total dollars and cents decision.
Try to find a company that has lots of uncleared work for you to do while you wait. FDA, FAA, Treeasury, EPA, NIH, etc often don't require clearance for a lot of their work and if you get the right contact within the company, that manager will often sponsor you for a clearance while you work on a federal contract.
Take a look at federal (non-DoD and non-IC) work contracts with SAIC, Nothrop Grumman TASC, Lockheed, Booz Allen, Mitre, Aerospace, Boing. AT&T, Scitor and General Dynamics. You'll probably have to move to where the bulk of cleared work is. Northern Virginia, San Diego, Denver among others.
Goto www.dss.mil and pull down EPSQ to findout what paperwork you'll need to submit to get your clearance. And forget about going for the Secret first so it'll make your TS/SCI easier. Totla BS. Each investigation is a start from scratch deal. Might as well go straight for the TS/SCI if you can.
Good luck.