Emulsion paint stored indoors, as it freezes. Oil paint is fine in a garage regarding cold, but if in a metal tin then condensation and rust are the main enemies.

Paint the lid with the last of the paint on the brush after you have sealed it, so that you can tell what colour it is years later. Don't rely on the printed colour mix sticker - it will fade. If you need to know the colour code, write it down in booklet of room colours and codes.
Also painting the top of a metal tin protects it from rust, to some extent.

If there is a skin over the paint when you re-open it, resist the temptation to charge in like an icebreaker, but make two holes and pour the contents off into another, preferably suitably smaller tin. Floating solidified paint won't dissolve - if you stir it in you just get bits and lumps in the next coat.
If you can be disciplined filtering paint with a cloth stretched over a funnel works well, but is very slow.

Never leave a brush in a jam jar of water or white spirit, unless you are literally only breaking for lunch. Paint will slowly build up in the bristles and spoil the finish. A professional once told me that when you pick a used brush out of your store, it should be impossible to detect that it had ever had paint in it before.

I decant enough paint to work with, and get the lid straight back onto the pot to try not to let the solvent or water evaporate from the pot, as edges will dry out or skin over. Don't dip brushes into the clean paint in the pot. Wipe the drips off the rim so they don't dry and fall into the paint next time.

Branded paint is sometimes much cheaper online (eBay) than in a DIY store.

No, I never put the brush into a paint pot, that's how dust gets in. Take the lid off the paint pot, decant a working amount into a clean container such as a shallow tray. Immediately wipe the drips off the lid and rim of the paint pot, and replace the lid firmly. It'll be good as new 15 years later when you need to redo the paint (I have found!)

If oil based starts to skin, in my experience, it'll always skin thereafter. Solutionn is to decant, with swringe or scoop (so as not to sully the tin lip/edges) and replace the lid - pronto. Keeps skin forming contaminatels (moisture??) out and paint lasts. Bit of a faff though.