I'm not a frequent flier like most of the people on FT, but I've learned a few tips from those hard-core road warrior types over the years:

1) Always put your carry-on bag above your own seat. If the space above your seat is already occupied, put your bag as close as possible, preferably in a spot that where you can see anyone accessing the bag.
2) Put a lock on your carry-on bag to prevent rifling.
3) Select a nondescript carry-on that doesn't scream, "I am an expensive bag with desirable contents - please steal me!" My own carry-on generally contains several thousand dollar worth of camera and computer gear; yes, a drop in the bucket compared to pro photographers and a few others who have to carry expensive gear for their jobs, but it's a lot to me! But that bag is a cruddy, cheap Samsonite roller that I paid $40 for at WalMart. It's a little worn-looking and doesn't look like either a camera bag or a laptop bag.
4) Remove anything from the bag that you may need to access during the flight, especially PEDs, and keep them in a smaller bag that will fit in your under-seat space. You're already keeping your wallet, passport, and Blackberry with you, which is smart.

If you are truly worried about your bag being stolen, you could make a habit of choosing seats near the front of the plane so that you can make sure the bag is still in its place when you land; if it's gone, you can block the aisle and inform the FAs that your bag has gone missing, and whomever took it won't be able to deplane till the bag is found.