I use a small coiled cable (combination) bicycle lock. Everyone has seen these in stores. They cost about $5. The only person the cable lock would/could stop is the 14 year old teenager that is bored or has a chip on his shoulder (and no tools). And that's OK. I don't leave my bike alone often... and there is very little that I could do (IMHO) to stop a serious thief. I'd rather a determined thief take dikes (side cutters) to my cable lock while I am in the men's room.... than bust my head open when I am not looking.