Hello and welcome.
First off shaking it kills the yeast faster, since the yeast is already killing itself when it makes the alcohol less help is better. The days i did do DIY C02 i found it very nice to run multiple bottles (at one time i was running 4) staggering the weeks you make the new solutions helps to keep a constant supply of C02 for the tank.
My best secret was to keep the yeast warm round 75deg as i found it produced more Co2 though it did not last as long. I did this with a small bulb in a fixture( that was a hard way to do it) then i found a tank heater mat for reptiles that worked very well.
I strongly suggest the sticky
http://www.aquaticplantcentral.com/f...ad.php?t=14453 Its a good read and very helpful.