From the Warp asked the hobby blogger community to submit "your one best painting tip." Mine isn't strictly painting but since it's my most shared tip RELATED to painting I'm putting it forward for the thread.
The photo is a three cavity painting tub. I picked this one up at Michael's Arts & Crafts in the USA for $1.99 -- although I wasn't able to find one on their website. It is, without exception, the only thing other than paints, brushes and models that I think every hobby painter MUST OWN.
This will extend the life of your brushes dramatically. Add a bit of brush cleaning soap - some people use regular dish soap - in the big chamber and it's even better. If I had to give up all of my fancy gadgets and tools but for one this is what I'd keep.