For example, if you ask for the Boomer support during Render Unto Caesar, you will gain massive NCR infamy. If you aren't at least liked with them, your reputation will turn to shunned or worst and NCR personnel will shot you on sight. But asking for Boomer support renders a lot of Legion fame, wich in turn makes you liked with them (if you are playing as a woman this is important, since you can't fight in the arena). So you ask for Boomer support, report to Caesar, gain NCR infamy, then go to the Ambassador and do the Boomers quest for NCR, reseting your status with them (actually it won't bet reseted, you will have the lowest possible helpfull "good" status, Good Natural Rascal. From this level of reputation you can do good deeds until becoming again liked or idolized).
Or do the quest "Booted" for Boone, while trying to maintain a neutral reputation with Legion (this is a no brain strategy, tell Boone to wait at Ulysses ranch, use a stealth boy to kill all the legionaries at the camp - and collect their ears! - then go back and bring Boone to free the prisioners).
Or do the quest We Are Legion before going to the Fort without failing Return to Sender, Medical Mistery and No, Not Much.
It's easy now that I know what to do, but at first you have to plan your steps carefully.
One thing I like to do is planting the bug at Cottonwood Cove for Astor, then asking for Aurelius trade NCR dogtags for supplies, then going with Boone to the overlook and kill only him without a stealth boy and without gainning Legion infamy, for Boone's points on his quest.
Ok, this last part doesn't count, it's a personal challenge I impose myself.
Killing Silus for Boone's point is lame, I love listen Caesar complains about him (that's why I kill Mr. House before gave him the Platinum Chip if siding with Yes Man, the note he gives you is very funny).
But you got the idea.
[ ]'s