The way to learn a tough lick is first to be able to grasp the pattern or shape that your fingers have to do crossing the fretboard. Trying to work out the tab without being able to see the pattern and rhythm in a particular run is next to impossible. After that it's just a matter of playing it slowly then building it up to the correct tempo. Playing guitar is similar to sports in terms of the learning process, visualisation helps a lot. It has been proven that if your visualise yourself doing something over and over in your mind before actually doing it, you will be able to execute the action more accurately. Sounds a bit far out but it works, I have very little time for practice at all these days so I use this method a lot. Of course you have to practice playing the licks too!