I recently had a custom built turbo back 2.5" exhaust put on my car.  Since the exhaust is solid I had to do something with the motor mounts to keep it from flexing and breaking.  I was just going to purchase some urethane inserts for the OEM mounts. However, my friend told me that he had filled his with some decent results, so I decided to give it a shot.  I started with the front two engine mounts which are covered in this article.  After removing the mounts and brackets on the engine I first put some large shrink wrap around the bracket shaft to make it fit snugly in the mount.  A garden hose would also work for this task. The reason this is done is to because there is usually about a 1/8" of free play in that area.  I then used 3M brand window weld to fill the mounts making sure to leave no air spaces.  To accomplish this I used a box with a hole in it to lay the mount on.  I covered the box with wax paper to make the adhesive release a bit easier.  I filled one side of the mounts and then flipped them to make sure all the spaces were filled with adhesive.  After letting the mounts cure for about 30 minutes I put them in the oven for on the warm setting for 35 minutes.  This made any air inside the adhesive dissipate to form a completely solid mount.  Once removed from the oven I let the mounts sit for about 24 hours.  At this point I felt the adhesive was cured enough to install the mounts, though, I did noticed over a few days they seemed to get even more solid while on the car. 

{gallery}enginemounts{/gallery}