Decision Point has an interesting article on share trading mistakes, which provides a really good starting point for things to consider when investing in the stock market. If there was a share trading course at university, this should be used as its first lecture.
One of the points the writer puts forward is that despite the stock markets out performing most other financial products over time, most casual investors do not invest for long enough to see the benefits (and therefore leave the market with losses). Emotions tend to get in the way, and the majority of people start pulling out when they begin to see losses – in actual fact, these times are often the best times to start buying.
The article also gives examples of several other investing mistakes to avoid.