Grind the activities until you reach x40 respect. Once you hit it, you have infinite respect. It makes the story a lot more enjoyable. Also, here's the order I recommend you do the gangs:
Vice Kings
Westside Rollerz
Los Carnales
If you leave the Carnales to the end, you'll be able to blow through them more easily. Oh, and make sure to buy clothes, etc. as they help your respect out too. The Drug Trafficking gives good respect, as you kill lots of cops too, which all adds to respect.