How to gain NCR reputation?

I was once good friends with the NCR in New Vegas, but I killed one veteran ranger for his armor. I wanted to get rid of Dark Hero rating so I could use their radio, so I did their quests. I fixed forlorn hope, killed fiends, fixed bitter root, took Nelson and did every mission I could find, but my NCR rep hasn't moved. Is this a glitch? Can I fix this? Or is there some glitch to gain NCR rep fast, or lose my bad rating? Thanks.