90% alcohol from Walgreens. ... and a toothbrush.
Before the alcohol dries, you need to dry it with a dampcloth. Otherwise, the flux that you brushed and got dissolved in alcohol, will remain on the board after the alcohol evaporates... leaving a film mess.
Sometimes it takes a second scrubbing (and helps if you also brush all the alcohol off the board, so it's taking with it all the dissolved flux)
Remember, before the alcohol evaporates, it needs to be patted down with a cloth. That's the trick for me.