Please don't be too judgmental regarding this post because it may sound so simplistic that it appears that I've completely missed the point of the discussion (here's hoping that isn't exactly what has happened). I have approached lip slurs as exactly what they are, playing two notes in succession w/o using your tongue. Obviously this becomes more difficult as the interval between the notes becomes larger (much easier to slur between c and e than c and high c). In my mind the task becomes easier if you approach the problem by playing the lower (when sluring up) note then pausing...and playing the upper note. This is not a slur yet but builds a foundation for practice. While you do this analyze the feeling you have on each note. Next articulate the first note and play the upper note with out a tongue (still pausing between notes). Practice in this fashion until you can PLAY each note in succession w/o a pause (that is w/o a break in the flow of air). The trick seems to be not in the slur but simply playing to tones, regardless of the interval between them, without interupting the air flow with the tongue and without "shooting" for the higher note. For me relaxing seems to be the key. This is very hard to explain in words. With that said if I don't work on intervals I lose this concept and ability very quickly. Hope this is not too weird. Don't judge me too harshly, like I said, it seems like a very simplistic approach to a difficult technical problem.