I would advise all rat owners to teach their rats a "come" command so that if they do run off, it's easy to retrieve them. All of mine are trained (or are in training) to come to a clicking sound, "tkkk tkkk tkkk," since they recognize that much more easily than a voice command.
It's very easy to teach this - first, pick a sound that you want to use. Then, give your rats a treat in the cage, paired with this sound. Essentially, you're using classical conditioning to teach them that the sound means food. Once they'll consistently respond to the noise, you can start doing this outside of the cage. Also, then you can gradually move farther and farther from your rat, teaching them to come to you from a distance. Once they've learned this command, you only need to give them treats intermittently to keep them trained.
For treats, only use a tiny bit of food, like 1/3 of a cheerio, or a droplet of fruit juice on your finger. That way they won't get to be fatties.

Also, mix up what you use for treats, so that they don't get bored with a particular food.