Well, sometime basic info like this that we all consider common knowledge, may be so old that it has become buried and hard to find. So I'll go ahead and cover it one more time.
Lowering the body- Ok, so the body is attached to the chassis with tabs and slots. You can either raise the slot, or lower that tab. In the beginning I think most people slot lowered their bits. The easiest way I've found to do this is take a cut off wheel for a dremel, and use a pair of scissors to cut it into a smaller circle about 1/4 inch in diameter. After you put it on your dremel you can spin it against a file to get it perfectly round again. This gives you a smaller cutting wheel to get up inside the body. When you cut your slot, the round shape of the wheel leaves the ends of your slot rounded. I've found glueing two thin strips of plastic on either side of the slot will keep the body centered.
The way I believe most people do it now, and the way I prefer is to lower the tabs on the chassis. In the front its the same for a Zip Zap or a bit, but in the rear it is a little different for a bit. Either way, you gonna want to take hobby knife, like an Exacto and cut the stock tabs off just enough to where the body slips down. What's different on a bit is, you will have to leave enough tab to where the motor's heatsink will still have something to latch into. Which means you will more than likely have to shave out the back of the body to clear what remains of that tab. Its not the easiest or fastest way, but the best way I've found to do that is use your exacto blade to scrape that area away. (Notice I said "scrape" and not "cut".
) Once your body slips down to where you want it, I stick the exacto blade in the slot to make a mark on the chassis where I want it. Then you want to take strips of plastic (I used to use zip zap packaging, but the inconsistant thickness made my bodies crooked, so now I use credit card plastic) and superglue them to the bottom of the remaining tab until they come down to where you want the top of your tab. The way I get my tabs the right width is to cut a strip at the approximate width, with an ever so slight angle. I stick the skinnier end in my slot, until it gets to a place where its snug. I mark it at the edge of the body, and cut it off there. Then when I glue the tab to the chassis I make sure that's the end that goes into the body.
Those are the two most common methods, but remember you're not limited to the stock slots. You could fill the slots for a more realistic look, and attach the body with velcro, screws, whatever your preference.
Wild bumpers- Not sure what you're asking. I'm assuming you mean the body kits people do, such as front spoilers.
If that's the case, then most people superglue strips of plastic to their car. Use something like an emery board to shape it, and some fine git sand paper to finish it. Other people use putty for plastics such as they sell for models at most hobby shops. Again sanding away slowly to shape it. Putty can sometimes break off. A good technique is to start with the plastic to rough the shape out, and use the putty to help smooth it over.
Just like before you are not limited to these techniques. Whatever works for you, just be creative, and hey, if you figure a different way to do it, we'd love to hear it.
Ok that's the last time I'm going over body lowering for at least 365 days. the next guy who asks will have to do a search.