In more than half the states, there is a statutory obligation for survivors to honor the written wishes of the deceased, to follow your personal preference. In a few states, your wishes may be over-turned in the case of cremation, however.
Courts routinely support all but the most outrageous wishes.
Perhaps the most useful laws are those permitting you to name a designated agent for body disposition. Or perhaps one of your children is a little more inclined to follow your wishes than the others. Sometimes circumstances change, and it seems appropriate to change funeral plans, too. One woman's father lived so long that none of his friends were left to come to a viewing. Or perhaps you planned on body donation to a medical school in Idaho but die while on a trip to New York.