The best way to do the lemons is tree ripened and then immediately after they are picked. If this is not possible, second best is immediately after they are juiced.
This time of year, I use a frozen juice I produce, and then immediately freeze, from the lemons I pick during the season. The juice was not as good as I would have liked as we had a warm spell followed by a very unusual cold snap that froze the trees.
I salvaged as much of the best as I could. I got somewhat lucky though in that I found another few local trees the were spared from the freeze and did end up getting some good quality juice. I have enough frozen to get me through till next season.
Next year I am going to be a little more proactive than I was this year. One less thing I have to purchase.