What Akahma said:
I try to just change one thing at a time between tests. I've mostly focussed on just dropping countries and publishers that have 0 conversions with 3x offer spend, but if a country or publisher is red but getting conversions I'll keep it around for a while unless the ratio is off the charts (I usually drop if it's worse than 10x the offer value if it's converting). Once a campaign breaks even with that process I'll then top it up so it runs for 48 hours straight without touching it again, just to see what difference a bit of time makes.