For whatever it is worth, I will post some things that I have learned over the years.
It is much easier, at least for me, to make good buy decisions than to make good sell decisions. I have often found that what I sell does better that what I replaced it with. It is another way of saying what Warren Buffett says, "For investors when motion increases returns decrease." It also shows the randomness of returns from the markets.
Investing is for brainy people what golf is for athletes. Just as golf has humbled many a great athlete, the markets teach humility to those who think they can outsmart it. It seems that whenever I tell people that I have had success investing, the markets show me that I am not as smart as I think I am.
Boring is good, boring that pays a dividend is even better. What can be more boring than indexing? Boring works because you aren't trying to outsmart anyone. You have very little in transaction costs and you don't have to worry about what to buy and what to sell. Get a plan, stick to it. The next to most boring strategy to indexing is value investing. If you are going to try to beat the market, value investing gives you your best shot. It takes patience, but it is fun to see the dividends roll into your account. Value has a shot of working because you are buying the unpopular, unloved, and underfollowed segment of the market.
Bear markets are really where you make your money even though it doesn't feel like it. If you will buy when others are selling in a panic, you will pick up shares at a bargain price. It also helps a lot not to sell when the markets are down.
I am not a believer in market timing but I think you need to pay attention to valuations. You can look for good opportunities to rebalance or to buy undervalued assets. I have refused to chase hot assets and pounce when buying opportunities present themselves. I have rebalanced or pounced on opportunities maybe four times since 2000. Three of the decisions worked great, one not so great.
Pretty much successful investing is keeping yourself from doing really stupid things. I have my share of dumb things I have done but fortunately I learned from them. Having a written plan really helps in minimizing the dumb things we are all tempted to do.
I had some success at my favorite mutual fund company by focusing on mid-cap and smaller companies. I try to keep 38-40% of my stocks in small and mid-cap stocks. I was very happy to see Mel Lindauer talk about the "unloved mid-caps", I am not alone in seeing this as a rich area of the market.
Keeping expenses low is something that I have always had my eye on. I started buying index funds in mid 1990's.
I get a big kick out of the engineers and quants that come to the forum armed with statistics, ratios, and graphs. I think the math is very important but what we are really trying to do is quantify human behavior. It can't be done precisely. If people are in a panic, they could care less about the standard deviation or P/E or whatever of a stock. They will sell. The stats, graphs and that are useful and give us an idea of what might happen in the future but human behavior is unpredictable. I am more interested in the behavioral aspects of investing. Jason Zweig's articles on Your Money and Your Brain are very interesting to me. Quantitative analysis has its limits.
Slow and steady wins the race. Investing consistently and systematically is vital to your success. You won't be a good investor if you aren't a good saver.
A fool and his money are good for business.