Trout are naturally wary creatures and require a cautious approach. It is sometimes necessary to crawl (like a tiger) or even slither (like a snake) into position. River fishing teaches you heaps about approach.
Shiny rods, bright clothes, bad boat-craft and wading all scare fish. Matt varnish your rods, dress like a tree for bank fishing (if you are in a boat, nothing is more likely to scare the daylights out of a trout more than seeing a flipping great tree drifting towards him - when in boats dress like a piece of sky), keep your feet dry, hide from the horizon and keep low, especially in boats and keep your false casts to a minimum.
It is not always essential, but it usually accounts for a few more fish and every once in a while, all of them.
Another great fish scarer is floating nylon. The light shines around the mono and puts the fish off completely.
The solution is to use a leader-sink compound. You can make one yourself using a mixture of fuller's earth (available from chemists) and washing-up liquid.
Some people advocate using fish-slime. I can see two problems here: the first is that you have to catch a fish before you can do this, the second is that the leader remains shiny.
Mix the earth and liquid to a paste and use frequently. With dries, it is not uncommon to have to muddy-up, or degrease, every five casts or so.