just three ways:
Use dxf. This makes sense if you are e.g. working with AutoCAD. Get Georg Mischlers dxf2rad (www.schorsch.com), which is available for a lot of platforms. When exporting, take care of the units (I have always 1unit=1meter, so the same as in most radiance apps). Use different layers for materials. dxf2rad will use the layernames as modifiers in the exported rad-model. Than write a material-mapping file, using aliases. That way, you can use ONE material file, that you will re-use for all your projects (simply adding new materials when needed) and put all the layer-to-material information into the mapping-file, that will be valid just for the model. If you are working on Mac or Unix, make sure to encode the (ascii)-dxf file in the ms-dos world format.
Use obj. This is available as an export option for different 3d-packages. It is supported by obj2rad and the new mesh primitive in Radiance. Be careful, the coordinate system in radiance is with the z-achis up! Many of these modeler use other coordinate systems! The obj-export will often be triangulated, but you can use surface normals (e.g. for smoothing) and texture coordinates. As such, I would always prefer it for the not-so-classic architectural models containing lots of curved surfaces.
Use 3ds. This means that you first convert to mgf, than to rad. You can get "smoothed" surfaces like with obj, and 3ds is available in many 3d-modelers, too. You can get some simple material definitions based on the 3ds-file, so you should be able to view the exported rad-file without changes by objview (the other formats' importers don't use the material / color information). However, some of the advanced features radiance offers with obj now are not available with 3ds (tecture coordinates).
If you have a big model, and used some kind of blocks (symbols, references, however your cad calls them), you can try to import them as instances or by using xform. This might help to reduce the memory needed during rendering a lot, however, it adds complexity to the import. Maybe we will have a converter that can handle blocks and export them as such to radiance without user interaction (I read that an early radout release was able to do so?).
Be careful with some CAAD apps. Archicad e.g. tends to include all details in the model, and if you want to render the overview of a large building, you won't want all the door-locks and furniture in your model. So try to control the details as needed (use layers etc, assigning the void modifier can switch off objects, right?).
What applications are you working with?
I use dxf for most architectural models, obj for free-forms (curved surfaces, smooth surfaces). U used radiance with AutoCAD, ArchiCAD, FormZ, Vectorworks.
Good luck, CU Lars.
Lars O. Grobe