The first two of the following four methods involve adding insulation to the outside of the roof, so will be suitable if there is little headroom underneath, or if access is difficult. The second two involve adding insulation underneath the roof. If insulating on top of a flat roof, make sure that it still drains well so that water does not pool on top.
Upside Down or Inverted Roof:
An 'upside down' roof uses waterproof insulation on the outside of the building structure. The insulation is laid over the existing waterproof membrane and held down with something - which could be pebbles, turf (for a green roof), paving slabs, etc. Suitable insulation materials will tend to be a bit more expensive, and include cork, foamed glass and closed-cell plastic foam. You'll need to check that the structure can bear the weight of the insulation and finish. This option keeps the existing membrane, but there is a risk that water will percolate through the insulation and so cool down the roof deck - causing condensation.
A warm roof will have the insulation laid over a vapour control layer (itself over the roof deck), with a membrane laid over the insulation and suitable finish on top. You'll need to check that the structure can bear the weight of the insulation and finish. If you are replacing the roof membrane anyway, then this will be a better solution than the 'upside down' roof (above), as it will keep water above the insulation and so keep the roof decking warm. You could still keep the existing membrane underneath the insulation if it would be difficult to remove.
The insulation is put between the roof joists. A ventilated gap needs to be retained between the top of the insulation and the roof decking, to avoid condensation build-up. It can be difficult to get adequate ventilation, so this method is often not recommended (for example, Scottish Building Regulations warn against it)
A method similar to dry-lining of walls can be used, with a plasterboard/insulation board added to the underside of the internal roof - below the joists.