i found somone suggesting hosting a race and running it with just you in the race so you get 1st everytime. thought id suggest the best races to do this with.
one is around the jail (i think its called criminal record) set it to 1 lap. takes 30 seconds and you should get just over 500rp. with a motorbike you dont even need to slow down as it a circle. just hold the gas down for 30 seconds and turn. will also boost your driving ability.
second is a bicycle race (on yer bike). takes around 2mins but you should get over 1000rp each time and build your stamina ability (very slowly?)
hope this helps someone. if you dont like grinding dont tell me about it, just move along. this isnt cheating, rockstar left the option in the game and if people wanna use it while the option is there why not? its our game and we'll play it how we want!