I hate to waste things, no matter how big or small. I can't explain it and sometimes it's totally irrational, but I just despise throwing things out. This includes throwing out leftover lemon juice. It usually happens when I've needed a specific measurement for a recipe and I've squeezed a bunch of lemons because I didn't really know how much juice they would yield.
So what do I do? I freeze it so that I can use it for other recipes at a later stage. I've tried several different ways of freezing leftover juice, but I've found the most useful way is to freeze leftover lemon juice in ice cube trays in tablespoon quantities. This way I know how much juice I have and they defrost very quickly.
Freezing doesn't impact the juice at all, and defrosted juice can be used instead of freshly squeezed lemon juice.
If you have a lot of leftover citrus fruit that you can't use straight away, simply squeeze the fruit and then freeze it. In this situation you might want to freeze it in larger quantities such as half cup and cup sizes. Pour the juice into plastic resealable bags, remove as much air from the bag as possible, seal, and then freeze. Make sure you write on the bag what kind of juice it is, and the quantity.
So next time you have leftover lemon juice or for that matter, any kind of citrus juice - freeze it.