The basic trick you are looking for is to learn to alternate single note melody with playing of full chords in such a way that the ear hears them together--the key to it is to move from playing rhythm guitar, where the important thing is keeping the beat, to lead guitar, where the melody comes first(but still keeping the beat)--

If you've been playing for a while, you probably have all the technique you need to do what you want already--there are just a couple things that you have to sort out-

First thing is that your strumming pattern is getting probably getting in the way of playing your melodies--a strum takes several counts to execute, and when the strum does something that goes against the melody, the melody breaks down--You have to simplify the "strum" which is really just an accompaniment, so that it doesn't interfer with playing the melody--a lot of times, this means just letting the chord ring for the active part of the melody, rather than keeping up the rhythm pattern--sometimes, you just forget the chord entirely while you play the melody notes-then go back to the chord strum at the end of the melodic phrase-

Next thing is that you probably haven't practiced playing your scales by themselves. If you want to play melody, first you must be able to play smooth and solid single note scales.

Start out with a G scale, starting with the open G string--pluck the scale with your index finger, and only your index finger--and play it to a metronome--

After you've gotten so that you can do this perfectly, start playing from the bass G, using your thumb on each note, but pluck the open G with the index finger. Same deal as above.

At some point, it will occur to you that you can fill in some of the chord notes while you are playing the scales without throwing your timing off--this is where you want to be--

As far as playing up the neck--there are three basic closed position scales that you can play anywhere up the neck--they all really come out of the open G scale--but that is another story--