Heelspur and training
Next to the stretching exercises earlier explained, it can also pay to strengthen your legs. Training the muscles in feet and legs (especially the calf) can support healing from heelspur/plantar fasciitis. Even runners or cyclists do not necessarily have strong legs. They may only be strong in a one sided direction. Performing exercises may strengthen the muscles then. Main remark is that you should be aware not to hurt your heels while exercising, listen to your body.
A much-advised training for the feet is to lay out a towel in front of you on the floor and to try and roll it up using your toes. When this goes well you can even put small weights on the towel to make it a bit harder. Similarly, you can also try to pick up small objects with your toes. Both are useful exercises, without any risk of hurting the heel.
A nice exercise that stimulates the tissue of the fasciitis is to roll a golf ball under your foot. Do this for 1-2 minutes and repeat this a few times per day. You can easily do this at work, simply sitting at your desk. This specific exercise is often found helpful.
Next to these exercises, strengthening the upper legs may be usefull. Several exercises are possible, like standing on one leg, lowering yourself by bending the knee and standing up again.
When in doubt it is always a good idea to consult a physiotherapist before doing these exercises. some gyms even have one on-site.
See also this video (courtesy of sportsinjuryclinic.net):