This is a good page where it shows the various major scale patterns, though there are probably 1000's.
# 5 is the easiest to me to remember. If you play this at two it is A major and you can see that the roots are at the bottom of each E string and in the middle of the G. Also the major chord sits at the top of the scale so it is easy to vamp off the 6th (barred with no pinky) the major, dom7, maj7 by just barring the 2nd fret and moving the little finger.
It is good to memorize the entire scale first, then focus on leaving out the 3rds and 7th, which is easy in this pattern because of the structure.
# 3 is my second favorite since it has a nice symmetry, only extends on four frets and barred on the top (middle three) notes is the V chord in the key. In A major this is at the 7th fret. The roots are not as obvious, but can be memorized easily since there are only two. Its a little harder to skip the 3rd and 7ths, but it makes a nice octave run from the A string root to the B string root. Because you can barre all the notes on the top fret it provides a nice anchor and an easy way to do pull-offs, etc.
When you are looking at scale patterns you want to locate the chords that are in the pattern for that key. This scale pattern is essentially the open neck where the C chord is, just barred, so all the chords you've memorized at the open position are here --- you just need to provide the barre. For example, in #3 the G is a D form at 7, which can also be played as a full C form (barred) using all the strings down to the low root and ignoring the low E string. D is the middle three strings on the top of the pattern (like you would play A at 2) and C is the F form at 8 or a fret lower barred is C (again the middle three strings, or with the pinky added for the any of the variants.). E minor is the Bm form at 7, etc.
With these two patterns you can always find one or two locations on the guitar to jam. In some keys obviously one of these patterns will extend into open strings so those are a little trickier to memorize, but make for some built in pull-offs since the open strings are already in the scale.
The next step that I did was to connect the two patterns. Using A major pattern #2 is the top of pattern #3 (at the bottom of the diagram) and shows the notes that bridge between #5 and #3. Being able to bridge between the two allows you to do extended slides from one note to another up or down the same string, most often to the root (or to the root of the next chord in the song), but it doesn't matter as long as it sounds good in the context (or is not a 3rd or 7th).
The other bridge is the one that bridges from #3 down to #5. Pattern #4 shows those notes. Obviously once you get back to the top pattern that is playable on the neck you've navigated the full octave and the scale pattern is repeated. So you can play the licks in one pattern, then move up or down an octave (if the pattern is available) and repeat the licks in a different octave. Trust me, no one will notice that you are playing the same notes. :) In the case of A major #5 is at the top and #3 is at 7. In the case of D major #3 is at 2 and #5 is at 7.
I have pretty much memorized the entire fret board in all of the keys and know where the roots are. In the case of an odd key like Db I just imagine D and slide all the patterns down the neck a fret. Almost all the music that we play is either in a major key or is the blues. True minor keys are only used in Latin music or classical. I, of course, love that style, but so far I've only memorized one minor scale pattern. I'll share that later, along with simple minor key chord progressions.
I have learned a couple scale patterns in the blues, which I believe I already sent you. A blues scale is already an altered pentatonic, so there are no danger notes, though obviously some sound better in the context of the song. One thing I wish I did better is to land on the root of the chord as the chord's change. For the most part, all good soloist do this as a matter of habit. I can do it if I try to remember the progression as I jam, but I'm too lazy usually, so I fall back on the pentatonic, occasionally throwing in some other notes or some blues riffs. If you just did this, in most cases, you'd sound like a pro. The next step is to memorize arpeggios and modes that might change with the chord changes, but that is for true professionals -- or people with too much free time.
The interesting thing about the blues is that you can stay in the blues scale of the key throughout all of the chord changes or you can change the scale to the blues scale of the chords as they change. Both approaches sound equally interesting and can be used interchangeable in the same song.
