On longer rides, I carry a drawstring backpack for snacks and a little tire pump, and I have a couple of those zip-tie kind of cable locks I bring with me in there. They’re no match for someone with a pair of bolt cutters, but if I stop at a liquor or convenience store to refill my water bottles, it keeps it from being too easy to walk off with my Tarmac. I also do the big chainring/smallest cog trick like
Steve B suggested, to slow down a potential thief a little more. I try to put it where I can see it if I have to wait in line to check out. A friend of mine only stops for fluids at supermarkets, and he just takes his bike in with him, never thought of that. If I need to pee, I look for public parks, construction site porta-potties, or if I’m in Ventura County, farmland porta-potties (usually on trailers for use by the farm workers). I rode up to Ventura a couple weeks ago and as it happened, the route I took was devoid of any kind of facilities. I was starting to wonder if I’d make it to the nearest park when I looked up as I was riding by the Buenaventura shopping center and saw they were setting up for some kind of event, and there, as if by magic, were two porta-potties they’d just set up. I climbed the dirt embankment up from Main St., stuck my bike between the two units, and found blessed relief. By the time the workers saw me, I was riding away with a smile on my face.
Thankfully, I’ve never had a near-theft or actual-theft experience. I hope my luck continues to hold.