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.
Whether to have a Brazilian Butt Lift or Buttock implants is a very common question for people seeking more curves. Both procedures are FDA approved and safe but have completely different approaches. The implant is inserted under general anesthesia and is done as a one time procedures. A Brazilian butt lift involves liposuction then multiple injections done under general anesthesia. This procedure feels a bit more natural and is done in a gradual fashion so you build up to the perfect desired look. If you feel you don't have enough body fat to be injected, then the implants may be the way to go. Be sure to consult with a Board Certified Plastic Surgeon when scheduling surgery.