Mild jalapeno varieties vary in their heat levels and in their tendency to occasionally produce hotter fruit. Anything that stresses the plant will cause a mild Jalapeno, such as 'TAM Mild,' to produce hotter fruit. Keep them growing vigorously from seed to harvest and you will have the mildest fruit the variety can give you.
A very good way to reduce heat in a harvested pepper is to remove the inner seed walls. When you slice a pepper open you will notice some white membranes extending into the fruit interior. These are where the seeds are attached and where the substance that gives peppers their heat, capsaicin, is concentrated. Remove these membranes and the seeds and you will have a greatly reduced amount of heat. Boiling or roasting will not do the trick.