Nothing is more exciting than seeing a fish strike your fly in surface film. I can remember more than one occasion using a sinking dry fly  and the fish wouldn't bite. Get the fly floating again and the bite continues. 

Casts should be made quartering upstream and mended to keep the current from pulling the fly under as it travels down stream. 

The fly is fished as a dead drift with no tension on the fly from the line. 

The fly used should imitate the local hatching bugs,  a attractor pattern (royal coachmen) or a terrestrial pattern (hopper). Fish will strike the fly even when there is no hatch in progress if you can trigger a natural feeding response in them.

Hook setting tip for the dry fly strike. Pull opposite the direction the current is taking your fly line level with the water. Do not set the hook like your going to cast the fly. That introduces slack into the line defeating the purpose of the hook set. If you use the surface tension to your advantage you will get more hook ups.

Different types of strikes on the dry fly are the rip and the sip. The rip strike is when the trout slashes at the fly. The sip strike is when the trout gets behind the fly and sucks it in. The sip strike takes a delayed strike to get a solid hook up. On a sip strike say "one alligator" then set the hook. Sip strikes are typical for larger fish on the dry fly. I usually miss this strike the first go around, but remember where the fish is located so I can get a second chance  :-)

If you miss a strike on the dry fly, cast immediately to the same location and be ready. If you miss again or the fish doesn't strike, be patient and allow the fish to settle down then try again.