I carry a mini maglite with me in my bag, it's small and light but will give off enough light. Here's another tip I picked up, you should do this anytime you get a flat but is especially helpful in the dark, if you wear gloves take off a glove and use it to glide through the inside of the tire, if there is any glass you'll hit it with your glove. This is of course only helpful if you were gloves or have some spare piece of cloth with you.