There are a few key points one must consider when trying to composite 3d onto live action.

1. The position of the camera.
2. The position of the 3d object in relation to the real space.
3. The adjustment of the lighting in a realistic manner.
4. Adjust the surface of the ground plane to blend with image.
5. Making the shadows match others in the image.

To begin load the files from the folder comp stuff:

dude.lwo
face.jpg
simple plane.lwo
subway.jpg
subwaybegin.lws


As you can see this is a really basic setup.
One of the first things you'll want to do is set up the proper camera position to match the angle of the camera in the image. Under the scene tab, effects, click on backdrop and add in your subway image. Now go to your display options and set the camera to show the background image. (see fig 1) Using the size of Dude for reference and other people from the image you can estimate where the camera was placed. When it comes right down to it gut instinct is the best way to tell. If the angles dont work then you will notice it, just do your best to make sure he fits with his surroundings. One of the things you can do to help you is to change your main view so it shows bounding boxes so you can see through the ground plane to properly line up your shot.

Next you want to go to the surface editor and highlight the simple plane surface. You're gonna want to add a image layer to the color setting. Load the subway image into the image editor and set it to front projection and turn off pixel blend, and texture antialiazing to not mess with origional image. (see fig 2) Now make sure your ray trace is on, turn on your image viewer and render out a test pic. It should look something like this (see fig 3).






Figure 3

It should be starting to come together. Now your going to mimic the lighting setup that is in the subway. You want to make sure you have at least one key light to provide the major shadows in the scene and then several fill lights to simulate radiosity. Reference Adam Sanders tutorial page on lights here for help. I'm using a quick and easy version of his setup and then placing random lights throughout the scene to match the light sources that I can infer would be there combined with lots of tinkering with light color and intensity.(see fig 4) Experiment to see what works best for you. Just remember ray tracing is a killer on render time, so if you can go with shadow maps then do so.

Depending on the image your using, you will probably want to resize the plane surface in order to properly line up with the beginnings of walls and drop offs.(see fig 5) If your object is going to react with other obects in the scene that may cast shadows on them, you will have to construct those objects in lightwave and treat them the same way you would the simple plane to make them blend in.

Now that you have the lights set up how you like them then go back to the simple plane surface setting and play with the luminosity and the diffuse until you cant tell where your plane ends and the image begins.

Just render out an image to see if you've got it matched up.(fig6)

The best way to get good at this is just like anything else, practice practice practice.