Obviously having a friend around would be the best solution, but if you're out by yourself and nature calls/screams/wails, what is the best advice or device for preventing bicycle theft?

I wouldn't want to be carrying a hefty lock around, so I generally have a brightly colored wire-lock when I'm out on my Dahon. As long as someone cannot just jump on my averagely priced bike and speed off, I'm okay with the minuscule risk of someone pulling out a bolt cutter.

However, it's far harder to ignore the consequences when it's a US$2,000 Bromptons or Bike Friday. Ideally it needs to have a loud alarm and some form of light chain to secure it to a fixed post.

What would you guys recommend?