I'm going to start this post now, and update it as and when...
Usually, I buy avocados for a recipe either too far in advance and so they go bad in the duration, or on the day and I buy them overripe and so they're also already going bad. So today I mashed 2 nice ones up with 1 tablespoon of lemon juice and some mixed ground pepper, put them in small plastic bags, getting rid of all the air, and froze them. They will keep for 3-6 months in the freezer.
This works well for double creams, mascarpone, creme fraich etc. Freeze in ice cube trays and use as and when. Brandy cream only semi-freezes because of the alcohol content.
Whenever I use a lemon for juice, I always zest it and add the zest to a freezer bag in the freezer. But, there are many times when I need lemon juice and don't have it. Freeze lemon juice as ice cubes and then put in a freezer bag.
When you buy a pack of these, it's worth cooking them all and then freezing them individually wrapped in foil, in a freezer bag. Take them out as required the night before. I chop some of them up before I freeze them too, to make preparation of some dishes even easier. In the UK, you can also but tubs of 50 chipolata ready-cooked sausages which are perfect for freezing.