I used to stick my bag in either the overhead immediately across from the Lav or under the seats in Row 13 (right against the bulkhead). I never had an issue with somebody fooling around with it. But then again, I didn't leave anything important in any of the easily accessible side pockets.
I wouldn't however leave anything else crew related in an overhead (such as a laptop). Those have been known to walk off. Usually you can spot if somebody is deplaning with a crew bag. It's much easier for them to conceal or otherwise smuggle other bags off.
Alternatively, you can slide your seat forward and put your bag behind your seat. I have flown with a few people that did that, but the downside is that it's hard to climb in and out, and you can't get enough room to put your feet up in flight.