Do mission on hard find some higher lvl friends too get you better missions.

Find someone that is high enough rank to invit u to coveted or pot shots. Both easy missions with high rp. Otherwise there are a few glitches that u can use. One go to sandy shores and find the los santos customs that is across the store from the market. (Must have car for this doesn't matter if it is your or off the street) walk up to store and punch air hop into car wait till screen stops flashing and then pull into los santos beforw cops spot u. U gain 200 rp every time u do this and can be repeated as fast as your character can move back and forth. The other is to find someone who can request a specific job from lester. I dont remember the name of it but there have been posts over the last couple days on it. Try scrolling through the pages and looking for rank up fast.