CO2 has been shown to quicken the flowering cycle. Keep all the enviromental factors at ideal with the temperature on the high side of the range.
Some people swear by using 24 to 48 hours at the switch from 20/4 to 12/12. Some strains seem to jump straight into flowering with no stretch first.
Almost any thing you do will decrease the quality and quanity except perfect conditions and CO2.
Also, there are some pretty quick strains.. AK-48/47 and such. There is a Train Wreck strain, called The Wreck that can be harvested in less then 8 weeks from clone and be at their peak.
Best of luck.