Okay, the thread title is a bit misleading. Essentially, my strategy when I play this game is to quickly figure out whether or not a stock is worth pursuing. I try to align the stocks into tiers:
This is the tier for stocks that I do not want. If two players have bought 3 into it before my turn after it was made...then I probably don't want to fight them for control (unless I'm trying to force them to buy more, or if I have the funds to outbid them). If three players are fighting over one, then I don't have to worry at all.
This is the tier where I'm not trying to have the most, but I will try for minority. Usually it's the one where the player before me built the company, but sometimes it's a company I built that I'm not investing heavily into, or it's a company someone else built that is largely being ignored. Being the minority here can sometimes prevent a super payout for the sole shareholder.
These are the stocks I focus on, where I want to be the winner. I will focus on these for the sake of getting the majority bonus.
This is all well and good when buying stocks, but the important thing is that after a company is bought out, I reset my tier system for that company. What I do with that stock is based on:
If the larger company in the merger is one of my upper tiers, I will trade into that company. This gives me a much easier time maintaining control.
If there is a lot of room left on the table, I am very apt to keep my cards. If I've done my job right, then I should be minority or majority on the buyout, and that gives plenty of cash in and of itself. If there is room on the table, then chances are the smaller company wasn't big enough to warrant selling my stocks. Keeping my cards makes it very likely I will be in the majority next time.
On the other hand, if we're late in the game and I have yet to invest in the merging company, I am most likely going to base my decision on what would get me the most money at the end of the game.
Of course, this is all subjective and based on your needs at that point in the game. The important thing to take away is that sometimes you can pick a company or three to largely ignore; it gives you more time to focus on ensuring victory through the others. At least, that's the way I play, and it seems to work for me.