Before starting, you'll need:
Floor jack, jack stands, 3M 08609 Urethane, 1/2"x9/16" socket, caulking gun.
As you know, there is a lot of rocking movement created when the engine is in gear and shifting. BMS make a very nice mount that works great - you can instantly feel the difference after its installed, and the install will take you all of 15 minutes. The mount is about $129 plus shipping. Here's a DIY that works nearly as well for about $22!!
I borrowed the idea from a previous car I owned that had a similar problem and a member of that cars forum solved the issue. You'll need to plan on doing this when you can let your car sit on jackstands overnight, but the wait will be worth it - trust me.
Jack up the rear of the car and place a jack stand under each side. I placed them under the frame tube. Then remove the two bolts and nut holding the engine mount in place. If you don't have the proper torx socket - I used a 1/2" x 9/16" socket and it worked fine.
When you get the mount out, you'll understand why you have all of the movement. Here's the stock mount. The rubber flexes esily around the mount because of all the air space.
Pick up some 3M 08609 Urethane which is used to seal car windows in place. Don't confuse this with silicone - this will dry into a much denser and harder rubber. I bought this at Advance Auto Parts
Open the tube, place it in a caulking gun and fill in all of the open space around the mount. Make sure and get it in all corners and air space on both sides of the mount.
Take a piece of plastic and smooth the Urethene out - forcing it into mount as you work it. and that's it!
Let the mount dry at least overnight and re-install back in your car. I used my floor jack to adjust the motor slightly so that the bolts would line up easier when I put it back together.
Is this as good as a BMS mount? Probably not, because theirs is a fabricated piece with a solid rubber mount. Does it make a difference in the way your Smart Car feels when you drive it? Absolutely!
And, you have only $22 into the project.