There are a lot of different things that will affect how an instrument will project over orchestras, not all of which we musicians have control over.
In no particular order:
Some instruments sound more than others, some instruments will be perceived even with a lot of sound coming from the orchestra. Il canone, the widely known violin on which Paganini played is such an instrument. I'll assume you have your instrument and will not be buying a 50k instrument for the sake of it.
This might well be the biggest influence on wether you will be heard or not. Great composers know how to balance melodic ranges so that even if many instruments are playing at the same time, the range in which the soloist is playing won't be filled with too many noise, allowing it to be heard.
There's no secret, producing more sound is possible. It has to do with bow pressure and bow speed, strings as well as the soundpost placement in your instrument. To choose the strings is a trial and error process, adjusting the soundpost is made with a luthier, and bow pressure and sound can be practiced with long notes.
- Knowing when to play louder
I once had the chance of hearing a masterclass with Vadim Repin. He told something really interesting, I don't remember the exact words but:
You have to play louder when it will actually make an impact. When doing long scales, you may relax the sound during the middle notes, but you will definitely put some emphasis on the notes just before switching direction in the scale, of when there's an harmonic change.
Regarding your comment:
Conductors expect soloists to have a sound that dominates the orchestra;
I really haven't had this experience in my life. As a matter of fact, all conductors were more on the side of telling the orchestra to play more piano so the soloist can be heard more.