Here's a quick run through using UV mapper classic. Yes do as Arianod described and tile by group or material. This will separate the model by groups or material. But you may now find the uvs to be to small for detail. So select by Groups or Materials,( Edit/Select ) and scale the parts ( Click & drag the dots of the selection outline ) Then drag the part to a place where it will not interfere with other part. You can also change the way any group or material is mapped, by using Edit/New UV Map while the part is selected. This way you can change the Axis it is mapped to, and if it is mapped front and back. and it's mapping style. After all the parts have been scaled, moved, and remapped, save the Model and the Map, Be sure to set the size of the map to be saved using Edit/Setting.

Stephen Ray