Practice a lot 'til your fingers bleed!
Here are some small exercises I found helped me to play some serious legato licks. Players like Joe Satriani, Steve Vai, and John Petrucci use these alot! These exercises are aimed at people who are already comfortable with legato playing. All these notes should be hammered on with no picking - this ommits any 'click' from the first note of each string and sounds great a high speeds!
They should all be played at 60bpm at first, then moved up by 5bpm and played correctly 5 times in a row - with no gaps! Your target should be about 180-200 bpm!
This should be played with six notes per beat.
e-----------5--7--8--7--5--------
b-----------------------------8------
g-------------------------------------
d-------------------------------------
a-------------------------------------
e-------------------------------------
Make sure that you hammer-on every note! If you have problems with open string noise, try muting them by reaching across and lightly touching the strings with your right palm - Satriani style!
Here's another at 4 notes per beat:
e---------------5--------------------------
b------5--6--8-----8--6--5---------------
g---7---------------------------------------
d---------------------------------------------
a---------------------------------------------
e---------------------------------------------
This effect sounds great for lazy legato lines drifting across chords!
Here is one that goes through the whole minor pentatonic scale. This one should be played in triplets.
e--15-12----12------------------------------------------
b---------15----15-12-15-12----12-----------------
g-------------------------------14----14-12-14-12------
d-----------------------------------------------------14---
a-----------------------------------------------------------
e-----------------------------------------------------------
e----------------------------------------------------------------
b----------------------------------------------------------------
g--12---------------------------------------------------------
d-----14-12-14-12----12-------------------------14~~---
a--------------------14----14-12-14-12----12--------------
e-----------------------------------------15----15-12--------
You should rememer this pattern as it sounds great in fast blues playing. You should also try basic finger independence exercises to make sure all your fingers develop at the same rate!
e.g.
e---------------------3-4-3---------------------
b-----------------2-3-------3-2-----------------
g-------------2-4---------------4-2-------------
d---------1-4-----------------------4-1---------
a-----1-3-------------------------------3-1-----
e-1-2---------------------------------------2-1-
Thats a tricky one! Move it up through every position all the way up the neck! And remember - no cheating! Don't pick a single note!
Finally, after all your hard work, here is a reward with a monster legato lick! I play this with 6 notes per beat, but if you're new to the technique try it with triplets first.
e------------------------------------------------------------
b-------------------------------------------------6-8----
g-------------------------------------4-5-7/5-7-9------
d-------------2-----------2-4-5/4-5-7-----------------
a-------2-3-5---5-3-2-3-5-----------------------------
e-2-3-5-------------------------------------------------
e----6-8-10/12-10-8------------------10-12-14-15~~
b-10----------------12-10-8/10-12-14-----------------
g-------------------------------------------------------------
d-------------------------------------------------------------
a-------------------------------------------------------------
e-------------------------------------------------------------
Take all these exercises slowly at first and then build up the speed - the philosophy I follow is "speed is a by-product of accuracy." Remember - no speed is too fast! These can be played at and up to 240 bpm! (useful info for any shredheads out there!)
I hope that helps.