I’m pretty new to blender, and I’m working on a simple motion tracking thing. However, it doesn’t look very good at all as is…I’m hoping that you guys have some useful tips that could improve it. I’m primarily looking for ways that I could make the pillbug (that’s what it’s supposed to be, at least…) look like it’s part of the background, right now it’s really obviously not natural-looking. Also any suggestions for how I could make the material better would also be appreciated. Thanks!
Fun idea…I’ve never attempted what you’re trying here so take these suggestions for what they are worth to you…
First, the shadow of your pillbug is cast toward screen right. All the cars in the scene seem to have the shadows directly beneath them suggesting that your primary light source needs to be directly overhead. You might need secondary light sources of a different hue to simulate bounced light underneath and sides of the pillbug.
Next, the scale of the pillbug might require a careful use of DOF to make the farthest edges slightly blurred (see cars at that distance.)
Other than that…I don’t know. Hope that helps get you along the way…
I would recommend making use of camera mapping to create an environment for the bug to catch light from the real footage.
you should set in the camera options the DOF setting ea camera aperature, so its as mus out of focus as those cars (use an empty, in front of the bug, and set that as the camera focus object; not the bug itself)
This is a challenging start. Kudos for not going the easy route, first off!
A lot of meshing digital elements with live action involves color grading and correction. Compositing should include several passes just to adjust for color and lighting differences between the digital and physical elements. The cars farther away from the camera are slightly faded as the light has to pass through more atmosphere to reach the camera lens. You’ve also got a pretty flatly lit scene. It appears to be noontime, or an overcast, cloudy day, and the lighting is very diffused, so the only shadow from the pill bug should be directly underneath.
Also, something that usually helps is to have the 3D model affect the environment. Having those giant insect legs leave small cracks or something in the pavement as it walks would help sell the idea, or at the very least, have flicks of dirt or pavement chunks and dust shoot up at each footfall.
But the biggest seller is the color difference between the bug and the live action elements. Try to figure out your digital camera’s specs, including the color settings that you were using, if possible, and Google them for some useful data you can then input into Cycles or whatever render engine you’re using.
Hopefully that helps…sorry, it’s before my coffee this morning. 
Dan
Thanks everybody! Sorry for the late response. Enabling and tweaking the DOF made the biggest difference. I thought that it wouldn’t do anything, because I’d already tried Gaussian blur without any impressive results, but DOF really made the difference somehow. I also was doing the shadow layer incorrectly, after learning how to do it correctly (from here) things started looking a lot nicer too. Putting the shadow directly under the bug also helps a bit, but not that much. I know that adding chunks of uprooted pavement or something of the like would make it look a lot better, but I can’t get it to look realistic at all. I’m gonna work on finishing the rest of my animation before focusing on details like that. Also, how exactly should I go about correcting the colors? What nodes should I use? Thanks!
In addition to what Dan said, your background video has a strong bluish tint caused by the overcast skies. In the compositor, add a Color Balance node to your bug layer and give it some blue tint. The three color wheels are Lift (shadows), Gamma (midtones), and Gain (highlights.) All three could use a little blue because it looks like there’s no direct sunlight.
Also, the background is a little blurred, but the bug is sharp. Add a little bit of blur to the bug layer (1 or 2 pixels is probably all that’s needed.)
Steve S