Keeping it cold is about the best way to keep a growler tasting better longer, otherwise growlers aren't not meant for long term serving once opened. Also, I usually don't recommend putting homebrew in growlers when bottle conditioning. I have had (and witnessed) several growlers explode. They will hold the pressure of a carbonated beer just fine (like off the tap), but when you put a beer in a growler then try to increase the pressure by carbonating, I have found a high failure rate. Potentially dangerous and messy in my opinion.