Investing principles are very simple and yet following them requires the will and determination of a rock, yes a ROCK, sit where you like, stay there and not be a smart ass.
First, Definitions.
I want to make sure you know I am talking about investing and not speculation or betting or arbitrage. Speculation is buying something anticipating some other fool will buy it at a higher price. Betting is trying to beat a random walk market with your data analysis. Arbitrage is trying to cash information/market inefficiencies, buying low at one place, selling high at another place. Money can be made in speculation, betting and arbitrage but I beleive nobody can make that money consistently over long periods of time.
Investing is getting returns in the long run consistent with your risk level.
Now to Principles.
1. What to look for ?
I like Buffet's principle on this one. Its the business, the people and the price. The business prospects should look good in the long run. Here is a crazy thumrule that I use, if that business goes bust, the end of the world is nearing. Softdrinks, clothing, chewing gum, automobile, insurance are all examples. Next the people. I beleive its the people you are investing in, businesses encounter challenging problems all the time and its the people who make decisions to steer it in the right direction. Ofcourse they should be smart, but more importantly they should be honest. The thumb rule on this one - they should be people with whom you want your daughter to get married (not the Indian father, for him having money is one of the top criteria).
2. Diversification.
Sure but what is it ? The goal is to ninimize risk - If you want to diversify business risk (risk for a particular business) then owning a sector or multiple sectors is fine. If you want to diversify for market risk, you have to get into fixed income securities.
For diversification, as academics say, the goal is to select investments which are inversely correlated with each other. Turst me, it is going to be very hard to find such investments in an increasingly global interconnected economy (remember Chinese stock market crash causing US stock market crash ?)
Interestingly, the best investment I found which is has minimum correlation with the whole set financial instruments is investment in a utility (food, medicine etc) company. not stock of that company but as a limited partner, where I would get to take out the cash flows annually.
3. Invest in what you know
If you want to invest in individual stocks or comapanies, invest in what you know. If you know a sector but cannot pick individual stocks, ETFs are there for you.
My Ideal Portfolio
low tens % - ETFs of what I like (Green, Emerging markets, REIT and high yeilds)
low tens % - Stocks (big cap and small cap)
low tens % - Public company stocks that I like and admire
mid tens % - Ownership in a private companies that produce good cash flows
single digit % - Fixed income securities