Prop balancing is the first thing you should do and I second the DuBro balancer as it works great. There are a couple others around but I have heard some bad reviews on those being off balance themselves with bent shafts and other such issues. For a few dollars more you may as well get one that works properly

Prop balancing is good even if you have a mount that reduces vibrations very well as it will avoid the insides of the phantom shaking themselves apart.
With the jello mounts, I had the VGE carbon fiber mount but found it a bit too soft. I had to mod it to be able t use it in wind or more then 50% speed so I wasnt real happy with it. I then crashed and this is the only part that broke, I then tried a DIY method which worked better at speed but had more jello when moving slowly so I ordered another when I got my FPV setup.
http://www.helipal.com/lp-0064-jello-ki ... -base.html This one use a similar system as the VGE but is better built. The carbon layers are thicker and stiffer, the rubber balls are also much bigger and have much less camera wobble then the VGE one had. I have only had a couple of quick flights with it and its better then the VGE was plus it also has some extra holes drilled where you can use the supplied cable ties to stiffen it some more or just to tether it in case one of the rubber grommits fails. On the VGE I was lucky my camera didn't fall off as one ball had nearly been cut through while doing some fast flying.
I also like how its cut away so you mount the DJI gopro mount inside the mount rather then outside to give you that little bit more space below the camera.
Also flying in bright sun it helps to have an ND filter to slow the shutter down and reduce jello this way, I use the Snake River Prototyping Blurfix adapters and filters when ever the sun is out. This also gives better motion if shooting slower frame rates in bright light by reducing shutter speeds and avoiding the stuttering type motion this can bring by smoothing it out with some motion blur between frames.
In the end though for best stability get a good gimbal and these will come with a vibration mount, the DJI one looks awesome and a couple others look pretty decent too but not sure about the real cheap ones as some of those look like a waste of money if they dont actually work properly.