Yak yak yak... Get a job!
You can start getting the various infinite ammo/damage immune/ubermench upgrades starting around rep level 41. The way to get them is to massage your rep rewards and side gains. First in importance is to always get the rep boosting upgrades as they are made available, and for the extra boost pick the option in the first branching mission that gets you another +10% boost. DO NOT take any of the 'spend cash for rep' options until after you have acquired and upgraded all three strongholds (Morningstar, Deckers, Luchadores). Do not to the 'Convoy' STAG mission or any of its following branch until you've gotten all three strongholds upgraded.
Vehicle Surfing. Get on top of a stopped car and balance yourself as long as possible while it moves. There's no limit on how many times you can do this for rep, and there's a challenge attached for total time.
Do all three challenges that involve killing gang specialists as soon as you feel capable of handling them. If you wait until later, you might not be able to find any of that gang due to control levels. When you do them, you get a good rep hit.
Same token, do the assassination and vehicle theft stuff early. A lot of the things you have to do to draw out your target can't happen if you control too much (I have 100% cover and have a bitch of a time finding Morningstar in the random street crowds... Luchadores should always be findable in the Quarantine Zone, however), and all of them come with a rep boost.
Survival missions... answer your cell phone when it rings and survive the waves of enemies at the marked location. Good rep, good cash.
Buy clothes... repeatedly. This one is incredibly cheap in terms of loopholes. Because every time you buy an item of clothing, you get its rep boost. So buy your beefy, manly man a casual skirt... 50 times.
Once you upgrade all three strongholds (total cost $420k), then buy the three 'cash for rep' upgrades. At this point you should have a +50% or more boost to all rep gains, so they'll be a massive boost and depending on how you've played might even have you in the mid-40's for rep. Here you can start getting some of the uber upgrades and will likely reach rep 50 before the heavy end game missions. And if you choose to chase after Killbane in the last mission, the chain it starts is very good to have as many weapon immunities as possible on.
