What you need for success
Because many of those who own unit trusts and OEICs are not very clued up about how to buy and sell their funds in the most effective way, it does create an opportunity for those who know what they are doing to profit.  The basic principles are simple and the rewards for those who get it right can be substantial.

As in any other walk of life, one of the secrets of success is not to let yourself fall victim to avoidable mistakes, but to spend a little time educating yourself on how to take advantage of the opportunities available.  Investing in funds is no different. Funds are, in the last resort, a known and convenient way to invest your money without much effort - but you do need to know what you are doing before you start.

Give someone a fish, as the old saying goes, and you feed them for a day. Teach them how to fish, and you can feed them for a lifetime. 

General principles
There are many textbooks about what sort of investment strategy you should employ, but my feeling is that there is a great deal to be said for straightforward thinking and common sense, rather than financial wizardry. Everyone starts from a different initial position. You may have a large capital sum to invest as a private individual, you may be the trustee of a pension fund or charity, or you might be trying to build up a capital sum for your retirement or for a specific purpose such as school fees.

Within those broad outlines, there will also be subdivisions. You might have had (say) an inheritance, have no need of the capital or any income for the moment, but you will want to take an income in the future. On the other hand, you may have urgent need of an immediate income from that capital, to the extent that you are prepared to see the capital sum diminish over time.  Whichever it is, make sure that you know what it is that you want and what you can realistically hope to achieve, rather than looking through rose-tinted spectacles and trusting to luck.

Mistakes we all make
Buying at the top and selling at the bottom.  By the time the least well-informed investor has heard about the latest 'dead cert' investment, it is almost certainly too late. The canny investors are selling their investments on to the unsuspecting 'mug punters'. The final stages of the technology boom showed this sort of behaviour, when people, often those who really couldn't afford to, invested far too much of their savings in investments that subsequently fell over 80%.  Similarly, when the phrase " abandon hope all ye who enter here" springs to mind, it is a good bet that the worst is over and that the most profitable thing to do will be, at the very least, to hang on in there, and possibly even add some more money to the investment.  This requires psychological 'bravery' and probably some patience as well.