Because they are usually pretty accessible, attics are one of the easier places in the home to inspect and insulate. And if there is no insulation, it's usually the most cost-effective insulation project you can do. Keep in mind the rule that you want to insulate any surface that divides inside living areas from outside spaces. Therefore in an attic that is not heated nor cooled, the ceiling should be insulated because it is the barrier between the conditioned and the unconditioned space.

Go into the attic to determine existing insulation levels. In addition to checking the area around its entrance, look deep into the attic's recesses. Sometimes insulation around the attic access is fine but not so good in harder to reach areas. This is particularly true with blown insulation. As you check the attic, watch your step! There is often nothing but drywall between the floor joists.

While in the attic, see if there is insulation on the attic door or hatch. These areas can be insulated by attaching a fiberglass batt to the upper side, or for entrances with pull down ladders, you may need to buy or build an insulated box to cover it. While it may seem inconsequential, the attic entry door is often a significant percentage of the total attic floor area. It should also be weather-stripped for a tight seal.
Unheated attics that have no flooring are the easiest to insulate. Simply add blankets, batting or loose-fill between the floor joists. Keep insulation back away from heat producers like recessed light fixtures or exhaust flues. Also, be sure the insulation does not block attic ventilation. A wooden baffle can be placed to hold insulation batts back from blocking the air passage. Sometimes plastic or styrofoam air channels are attached to the under-side of the roof to keep the passage clear. When loose fill insulation is installed, it's a good idea to clean the soffit vents afterwards to assure they are not blocked.

Insulation can be in the form of pre-cut batts or blankets inserted between joists or loose-fill spread evenly across the attic floor. Loose-fill can be carried up in bags or pumped up using a rented pumping machine. If the attic has a finished floor, loosen a few boards and blow insulation between the floor joists. Using anything other than blown insulation would require tearing up the flooring. If the attic has no insulation, see if vapor barriers are recommended in your area, and if recommended, where they should be placed.

If the attic has some existing insulation, you can add additional loose fill or apply another layer of unfaced batts at across angles to the joists.
Attics used as living spaces and having heating and/or cooling should have insulation in the ceiling and walls. Some of these areas may be readily accessible, but others may be covered with finished interior wall or ceiling materials. Check to see if there is insulation behind them. If there is none, your choices are to blow in loose fill from outside the home or strip off the finished wall and ceiling materials and install blankets or blown insulation from the inside.
Ceilings that are also roofs can be big energy losers. For types with attractive beams and exposed roof boards, one solution is cutting panels of rigid insulation and installing them between the beams. Most types of rigid insulation must then be covered with wallboard for fire safety. With ceilings considered too pretty to cover, insulation can be added during a re-roofing project.

If there is already some insulation in an attic or roof that equates between R-30 and R-38, it may be hard to justify the cost of installing more according to the principle of diminishing return on investment, in which case you can focus your energy efforts elsewhere.