Adding too much spice to a dish can ruin it, making it painful to eat for those who enjoy spice and inedible for those who don't. In some cases, you can eliminate some of the heat by pulling out large chunks of jalapeño, for example. If, however, you blended a spicy ingredient into the dish, you will need to add to it rather than take anything out to reduce the heat factor.
Add extra liquids to reduce the spiciness if this is possible and appropriate for your dish. If you made a soup that turned out to be overly spicy, for example, add extra stock or water to make the soup less spicy. This will also make it less thick, of course.
Add extra recipe ingredients to your spicy dish, if possible. Do this regardless of whether you were able to add extra liquid or not. In the soup example, you could add more of whatever solid ingredients were in the soup. This might include potatoes, meat or noodles. This will help reduce the spiciness in a dish to which you could not add liquid or help recreate the texture of the original dish if you did add liquid. While these items do not actually remove the spiciness, adding more non-spicy ingredients to the dish will help reduce the ratio of spice to other ingredients.
Blend a dairy product into the spicy dish if it would work well. If you made a spicy scramble with potatoes, cheese and jalapeños, for example, you could mix in some sour cream. If you were making a potato leek soup and accidentally sprinkled in too many chilli flakes, blend in some extra cream. The dairy will help tone down the heat.
Add granulated sugar to the dish if none of the above strategies would work. This method works particularly well with Asian dishes because Asian cuisine often combines sweetness with spice so the combination will not taste out of place. Start with a small amount of sugar -- such as 1/2 tsp for each serving of food -- and add more if necessary.
If you weren't able to reduce the heat in your dish, drink a glass of milk while you eat your meal. Milk works better than water, soda or other similar drinks to dissipate the spiciness in your mouth and make the heat of the dish more bearable.