I seen some nice photos in the user galleries were the build pics were placed on a background pic. I've played with it a few times but my results have never been worth posting. So I'm looking for any tips on how to do it.

This is the image I'd like to add something to:



I'm also curious to know if I've cropped out the old background properly.

Any help is greatly appreciated.

Well you have got most of the grunt work done by isolating the focus of the picture. What comes next depends entirely on the editor you are using and the background you are trying to add in. The tricky part as you have probably learned from previous attempts is making the background blend well with the foreground picture.
Generally speaking you want to make sure that both the perspective and the lighting are consistent between the two. Most image editors come with tools that allow you to transform 2D pictures into the proper perspective. You may notice that a lot of pictures from the gallery that have backgrounds are shot from a low point in the picture as to allow the user to render them with a horizon. Generally adding a sky with a low horizon is a lot easier than a more aerial view down to the ground.
For instance look at Greasius’s mars build

Note the low horizon. Also look at all the Village Works pictures as they nearly all have purely a sky background. The low shots also give a more dramatic view of the set by placing the viewer inside the set itself and are easier to render.
Lighting is a little trickier. Be sure to follow the model for a reference. For instance keep the shadows casting towards the right. Other than lighting and perspective the rest is just attention to details. For instance the black edges on the ramparts may clash with a bright background. A very small soft brush may be needed to touch up areas that are out of place or clash with the background.

LD...Your buildings look so good than you dont need a background for the pictures....Already I wanted to do constructions as clean as yours.

I take the point about using low angle shots. Sadly thats another thing I have trouble doing. How do you go about doing that? Do you use a stand? How do you get a good shot with a fairly crowded layout?

A stand like a tripod?

With crowded layouts, I usually don't glue things down. That way I can make "break-away" bits, like they do on TV and movie sets. It makes it easier to cram the camera down in the layout! Just set the camera on the "floor" of the layout, set macro mode and fire away. The pics generally look great, like you are standing in the set itself. Depth of field can be a problem, though.