yep, need access to cootonwood cove, hand in enough pick-pocketed ncr dogdags to boost rep with Caesar, speak to Aurelius of Phoenix at cootonwwod cove about this its a free quest called arizona scavenger, hope that helps.
also be nice to Vulpes Inculta when you meet him in Nipton, say the inhabitants deserved what they got and you admire his handywork. and when you get to 'the Fort' you can beat the Arena for more fame.
People need to remember to sign their post. -- Dopesmylez
Another way that this can be done, is to wait until after you have confronted Benny in the tops and received The Mark of Caesar (you will recieve it from Vulpes Inculta, unless you killed him in Nipton, in which case you will recieve the mark from Alerio) guaranteeing you safe passage to the fort regardless of your previous crimes against the legion. Upon arriving at the fort, try to only do quest that do not involve confrontation with the NCR military or it's citizens. With the combination of that and handing in NCR dog tags (an easy way to do this is to kill NCR feral troopers at Camp Searchlight that are not associated with the quest "we will all go together". The large majority of the ones you kill will have tags) to Aurelius of Phoenix at Cottonwood Cove, (as mentionted in the post above) you should acheive the status necessary to gain entry to the safehouse fairly quick. Also, you need to note that you only need a reputation of "liked" to recieve the key to the safehouse from Lucius, who is of course located at The Fort. -- Dopesmylez
There are a few people at the fort you can help for fame (Siri the slave girl, for one) and you can fight in the arena of course; none of this harms your status with the NCR. Also, the first part of Render Unto Caesar won't bother the NCR, and in fact you can do the opposite and lie to him and it's still considered a success - in order to proceed on the House or Yes Man track you pretty much have to, if you don't want to kill everyone in the fort.