Few tips that may work (and have worked for me at times, through trial and error)
-have spare change in your cup holder space or anywhere else that can be visible to clients but don't make it too obvious. The idea or hope is that when they see the coins, they will be reminded to tip.
-If you have a DVD player in the back, create a DVD with static images that change once in a while, with picture of nature, beach, flowers etc. Basically something calming. But here is the trick: using some photo editing software, you plant some hidden messages behind the picture. For example, behind the picture of a mountain, you can blend in text (not readable by the human eye) that reads something like 'I accept and appreciate tips' or 'tipping the driver is a great idea and ensures an even better service'
-Alternatively, you can have an audio CD that plays classical music but with hidden messages planted throughout. These can be found on YouTube privably