Cross training provides huge benefits to runners by allowing them to build and maintain their aerobic base when they are not running due to injury. When we are running, cross training is equally as important. It provides strength to stabilizing and opposing muscle groups. It adds variety to your workouts and you are less likely to get burned out. Last, it gives you an automatic go to sport if you are injured to keep you active and less likely to fall into the injury depression so many runners battle when they are unable to run.
You can pick any sport as cross training. The more opposing muscles that the cross training uses the more benefit you will see to your running. The idea behind cross training is to give your running muscles a break, but to continue to stay active and maintain that aerobic base you have worked so hard to achieve. If you are using those opposing muscle groups, you are also going to prevent injuries by balancing out the pulling force on weaker muscles when the stronger ones contract.
Cycling and swimming are excellent choices for cross training. Swimming focuses on your arms and shoulders as well as your core muscles. Your legs are definitely not the emphasis although they are used. If you don’t know how to swim or are a weak swimmer, I suggest getting a copy of the book “Total Immersion,” by Terry Laughlin. The Total Immersion (TI) method teaches you step-by-step the most efficient swimming technique. You can find classes across the U.S., but the book is designed to be used on your own. There are DVD’s you can buy which demonstrate each drill and skill outlined in the book.
Cycling uses opposing muscle groups (quadriceps more than hamstrings). Cycling also works the outer hips and the butt muscles. Making sure these muscles are strong, prevents hip rotation and potential illiotibial band issues. Swimming and Cycling are low impact sports as well, so they give your muscles and bones a break from the jarring of running.
Pilates and yoga are good complementary options as well. The elliptical or stair stepper are options, but very close to running and are more useful as a running substitute when you need to reduce the impact.
ultrarunningmom 
Its funny because when I first started running, I kind of scoffed at cross training. I did yoga once a week and yoga stretching after runs, but that was about it. I did fine running in that period of my life, but I always felt sore and tired and dealt with a few small injury issues.
Fast forward to this year when I decided to commit to a 70.3 and I’m swimming and cycling during the week (swimming is my weakest part, so I’m focusing on this more now in the early phases). Crazily, my running has improved very naturally and with very little effort. Pretty cool 😉
I did the same thing! After a few minor injuries mainly from overtraining, I started cycling and then swimming. I watch my building phase and take rest weeks now, and so far so good.