I'm living from design visualisation. Some thoughts to this topic:
* designers often are working under the pressure of tight deadlines
* I offer to work on the basic setup parallel to the design process, so that at the end of the project design model and visualisation setup are ready for the final rendering
* at the end, if the designers are exploited by creating the final model, I start the final high res renderings with a fine adjusted setup with known render times. the designers can take a break or use preview renderings for working on the final presentation
* your renderings must be highest quality, best beautiful

, so that the design office couldn't render it in this quality during a project
* you must be a specialist for rendering, get all effects done in shortest time (I use a biased render engine to get the max render speed)
* work for designers in your country - so you can fast communicate per phone or messenger
* make special prices for design students at the last semester. So, they got nice images and you get connections to design offices
After some time you could find some design offices and work together over the years.
An other direction of design visualisation are mass renderings for catalogs. Most, you get many detailed restrictions and you will paid per images and not per hour. I have the feeling this kind of jobs are done in Asia often.