It's unfortunate your main devices wont read cd-rw's. I tend to mix very intently on the monitors and then do a quick burn on cd-rw and check it the car, mainly for the bass response, not acurate - only a indication.
As the other guys have said, a ipod/mp3 player now is becoming a prominant device for referencing your mixes. I use my ipod mainly to check my panning and overall balance - again only as a indication.
Depending on what style your music is, if you have access to a club system, particularly a mono system, that can be another guide as well.
Perhaps investing in a device that can read cd-rw may be a worthwhile and time/cost (long term) reducing option
