If you browse the on-line patent data bases - either www.uspto.gov or www.freepatentsonline.com for "glucose monitor" you'll find there are at least half a dozen technologies that have been proposed including the most non-invasive technologies such as using light - similar to the pulse oximeters used in hospitals to measure the pulse and PaO2 through the skin to more invasive methods that periodically pump small samples of blood outside the body for chemical or optical analysis. But in terms of accuracy, the best way to measure BG is by some means that can analyze arterial blood in vivo.
Companies that are trying to develop these devices are shooting for accuracies better than 12 mg/dL, but that's very hard to do with the noninvasive methods. There is a very new technology that uses a contact lens as the sensor, measuring from the iris and transmitting to remote electronics for processing, but that's still probably year's away. I believe the best products that are available today may be those developed by Dexcom in San Diego, CA. See their website here: http://www.dexcom.com/
The journal Science just published its year end Breakthroughs and that includes an article that shows hope in restoring the B cells in the pancreas for type I diabetics see http://www.sciencemag.org/content/346/6216/1444.full . This of course is the best solution - a cure. I hold hope scientists are less than 5 years away from that goal.