Ask your self 'Is there anything hatching ? (and more importantly is anything being eaten by trout on the top)?

If there is, fish dry flies or pupas.

If there are no visible signs then fish under the surface with nymphs or lures.
Nymphs will be present most of the time as will daphnia. Dries and pupa will be there or taken during hatches or days when hatches should but can't occur due to the weather etc..

Ok this is really general (I've been at the vino too) and a good starting point but, like life, everything in trout fishing may prove you wrong

Just keep ringing the changes until something happens...
keep changing the fly colour, shape, profile, size, depth, speed, location (near the bank or out), direction (fan cast in front of you to cover all the water)...

Neil (ruddy roach: definately the best first question!)