There is a lot to say about this kind of question but as I am rather busy with other work I will just try and get the ball rolling here and then maybe other people can keep it going. So from a hindsight/hypothetical angle I could suggest a few things that may have helped...
The best chance of survival, although far from certain, would be an involvement with the communist movement before the civil war began. Soldiers and cadre were better off in Democratic Kampuchea and had access to resources or patronage networks that were unavailable to either 'base people' or 'new people'. This is a generalisation and is not applicable in many instances. For example joining the revolution from an area in the eastern zones of the country, those near the Vietnamese border, might see you become a suspicious element that the party purged in the later years of the regime's time in power. You were still subject to many of the same rules that dictated life under the watchful eyes of 'the revolutionary organisation' (angkar padevat) and any serious indiscretion could still mean death. Joining the revolution before the civil war would have ensured you enough time to bed in to the organisation and hopefully make some ties that could see you out of a tight situation if need be. But there were no guarantees and links to a person who came under suspicion of the party could also mean your arrest and torture/execution at a regional security centre or S-21.
As a 'base person', your life would be easier once the regime took control in 1975. You were able to forage and keep a small amount of food for yourself, in some areas, until restrictions from the part centre ended this. You would have been used to manual labour and working the land, which would have afforded your body less of a shock when the collectivisation and coercion of labour was implemented. You were able to 'get away' with minor indiscretions against the party with a warning in most zones. You would have been able to stay in your home village and maintain the patronage networks you had before the revolution and war, so you can imagine a scenario where a longtime neighbour may have helped you out in a tight situation or you may even know the village leader if he was promoted in your area. You would have avoided the massive evacuations of people after the civil war ended and were afforded a few more rights than these 'new people'.
Very important that you were not connected to any important members of the previous regime. This would have seen you killed almost immediately. Similarly and connection to the army or government agencies would have also been a death sentence if disclosed to the khmer rouge. Lying about these ties or any kind of 'educated' position would have been a must - a famous example is that of Dith Pran, who assumed the role of a taxi driver and 'acted out' this role. In an early checkpoint in the deportations from Phnom Penh he saw a cameraman whom he knew, the cameraman and his family thought nothing of telling the new regime of his role and connections to foreign journalism - he assumed he would be able to continue this job for the Khmer Rouge. Pran survived the checkpoint and his friend, and this man's family were killed. Having a good amount of trade-able materials would do you a lot of handy as well, not money, but precious things like gold or watches, there was a substantial black market economy in DK and these kinds of items could be used to buy certain essentials like medicine or rice if you knew who to talk to. Obviously that could mean a death sentence as well so, again, no certainties.
I feel like the thrust of your question is along these lines, i.e. what could your average urban dweller do to have the best chance of living.. unfortunately any answer I give will be cancelled out by some instance of an unfortunate person dying despite their best efforts. But, with that being said, it was certainly in your best interest as a 'new person', to really 'go-along' with the goals of the party.
One of the most fundamental aspects of Khmer Rouge ideology is the concept of satiaramma, which translates to 'revolutionary consciousness'. This 'new brand' of mindfulness relates to a person, or group's, ability to understand and implement the 'collective stance', of the party. This is in opposition to the 'private stance', that the Khmer Rouge were seeking to eradicate from the country - along with all the other 'feudal institutions' associated with the previous corrupt worldview.
In this sense it is possible to imagine a person just, fully embracing the insanity that Democratic Kampuchea was - obeying all the rules, spying on friends and family, working hard every day, 'doing the dance' if that makes sense. Being what the party wanted you to be, one survivor relayed a story along these lines :
“You see the ox, comrades? He eats where we command him to eat. If we let him graze in this field, he eats. If we take him to another field where there is not enough grass, he grazes all the same… When we tell him to pull the plough, he pulls it. He never thinks of his wife and children… Comrade Ox never refuses to work. Comrade Ox was obedient. Comrade Ox did not complain."
So, you could do all these things. But then you might be killed because a district chief had suspected that all the 'new people' in your village were somehow sabotaging the revolution, or perhaps someone named you under conditions of torture.
Anything you could do to avoid a condition where you might get sick, or hopefully have a body capable of working the excruciating amount that was expected of you would also help.
I wish I had more time to actually go through some of the biographies I've got and give you some better evidence of how they survived but as most of them would probably say the amount of luck involved is very large. Apologies for the kind of, 'off the cuff' response and if there are any other takes on this or questions you would like me to follow up on please let me know.