A slip in a pocket or keep in hand, the shape of tombstone fits well in the hand.
Flax seeds retain heat for a half hour to an hour. It is a plant with analgesic qualities, which helps to calm, lavender flowers also have a relaxing effect.
Flax seeds are rich in oil that helps maintain a gentle heat and soothing. You can also put them in the freezer (in a plastic bag to protect from moisture) under the front that helps relieve migraines.
Place hot water bottles in the microwave, about 1 minute 150Watts, you define the right mix depending on the power of your microwave, but never more than 2 minutes and not put too much (at the risk of burning your seeds) .
Personally, since I do not have a microwave, so I do heat the iron through the iron over a few minutes (be careful off steam). You can also let them near a heat source (radiator or heating oil)
The oven is also a possibility, but it takes about 15-20 minutes at 150 ° C and I think it's a lot of energy spent for nothing.