My trick with Stocks:
After the first mission (and I've played it three times because I keep wanting to mess around with different things), invest in one airline and destroy other airlines' vehicles. I think this might work if you invest in Cluckin' Bell and start holding up Burger Shots, but I haven't tried. If someone wants to get back to me about that, that would be great.
I have so many cars :\ At Michael's safehouse I have a Coquette and a Banshee, and at the garage in LS I have a Dominator (in Orange with black stripes), a dark blue Blista, the obligatory Infernus, and another Banshee because I made a bit of a whoopsie and I need to find one of those Audi R8-looking things instead.