I cover much of this in the Idle Problems article. I'll do a brief summary here. If you have a vibration at idle, focus on engine performance first. There might be a cylinder that isn’t putting out an equal amount of power, causing the engine to not run smoothly, or a mixture problem, or some other mechanical cause.
Don't assume right off that if you have a vibration at idle, your engine mounts are bad. New engine mounts bolted up to a poorly running engine means you still have a vibration; it's just muffled by good engine mounts. Start with engine performance and perform any necessary repairs. After you’ve done that if you still have a vibration at idle, then look to the engine mounts.
A lot of people tell me they've replaced all the engine mounts in their vehicle and they still have a vibration. Then I go through the speech above, and the cause ends up being a vacuum leak or something. Save yourself some time and money and look to engine performance before going for engine mounts if you have a vibration at idle. Some engine mounts are a real pain to replace and expensive; don't go there unless you have to.