the Great debate heard in fly shops online and on the water is what is most important the fly or the presentation. This debate can go on endlessly but to be completely honest both sides are wrong. While you can fish a perfect match of the fish are feeding on with a poor presentation and catch some fish or conversely fish the completely wrong fly with a perfect presentation yes you will catch some fish. If that is what you want and some people are satisfied with that it's ok. If you truly want to be in the 10% of anglers that catch 75% of the fish then put them together you will catch 5 times the number of fish you do now. 

Some people with go on and on able how the fly is not important as so and so catches all their fish on a size 14 Adams dry fly or so and so nymph. yes they may catch all their fish on them but how many are they missing because they are in the wrong current seam or fishing a dry when the water temp is 40 degrees and there hasn't been a hatch in 4 months. There a 5 or so things to consider when you are choosing what fly you are going to use. \

1) what are they easting? look for bugs in the are, on the water or in the bushes.

2) silhouette of what they are eating is next. Mayflies, stoneflies , caddis and midges all have different body / wing types in both larval and adult form.

3) size what size bug are they easting, there is a bigger difference than you may realize in a size 12 to 14 natural, the trout will get locked into a curtain size bug with an extended hatch. 

4)  finally proper color is also important especially during hatches like the Hendrickson and again during the Sulphur hatches later in the spring. it dose not have to be exact but close.

 where are the majority of trout feeding surface, film , mid current or bottom. once you determine that. how are they feed , flashes, bulging rises, splashy rises, slow head to tail rise or head bobs. These will tell you best where to start. 

Now you know what they want not you need to present it to the fish correctly, it may sound easy but sometimes it can be maddening. if they are feeding subsurface depth control is the number one thing to manage. If they are feeding up to emergers or adults then you have to get the fly to the lane the fish is feeding in. at that point control of your leader and tippet are A#1 to manage. Any drag will cause the fly to act out of the ordinary so it may put a fish down or cause a refusal. 

These above hold true no matter the technique you are using streamer fishing, swinging wets, dries or nymphing. So next time you are on the water try to put these together to see if it helps.