In most cases, you cannot use your will to leave:
For more information, see Property You Shoulde Not Include in Your Will.
Wills are typically not read -- or even found -- until days or weeks after a death. That's too late to be of help to the people who must make immediate decisions about the disposition of a body and funeral or memorial services. Instead, make a separate document spelling out your wishes and tell your executor where to find it when the time comes. (See Final Arrangements FAQ.)
If you expect your estate to owe federal estate taxes, you may want to take steps now to reduce the tax liability. A will won't help you avoid taxes. Many kinds of trusts can reduce or postpone the tax bill. (See Estate and Gift Tax FAQ.)
Property left through a will may spend several months or a year tied up in probate court before it can be distributed to the people who inherit it. (See Probate FAQ.)
There are also a few legal limitations on what you can do in a will. For example, you cannot leave a gift that is contingent on the marriage, divorce, or change of religion of a recipient. You can, however, try to influence lesser matters. For example, you could leave money "to Jeremy, if and when he goes to college." Making such conditional gifts, however, usually opens a can of worms -- who will enforce the will's conditions, and for how long?
This one doesn't come up often, but you can't earmark money for something illegal, such as encouraging minors to smoke.
If you want to provide long-term care for someone, a will isn't the place. Far better to set up a trust that's tailored to the beneficiary's needs. A special needs trust can provide extra income for a loved one with disabilities, without jeopardizing government benefits.
Nolo's book Special Needs Trusts, by Kevin Urbatsch and Michele Fuller explains how special needs trusts work and gives you the tools to make one yourself. Of course, if you have a complicated situation or if you would rather have an expert's advice about your specific situation, you may also want to see a lawyer who's an expert in this field.
Pets can't own property, so don't try to leave property directly to your pets in your will. Instead, leave your pet to someone who has agreed to provide a good home -- and leave that person some money to help out with pet-related expenses. Some states allow you to set up trusts for animals, but that's probably not necessary if you have confidence in the person you've named to care for your pets after your death.
Both Quicken WillMaker (software) and Nolo's Online Will (online) make it simple to name a caretaker for your pet in your will. Both products also help you leave money to your pet's caretaker, specifically for the care of your pet.