Injection may not be an option if you have no available fat to harvest.
I would stay away from injection of other substances, such as silicone, because of the potention for serious complications and even death from embolism. If you are slim but have some fat to harvest, usually even a small volume fat injection will make a significant difference.
If there is really nothing, buttock implants are an option and work well. They need to be placed deep, in the muscle or under the muscle fascia at least. You need to find a surgeon experienced with these techniques...