I've reached a plateau and don't see any improvement
This is common. It happens to all of us from time to time. Sometimes we notice fast improvement and sometimes we may stall, or plateau, for a while. Just keep playing, and youíll eventually break through it. If you're seriously stuck, however, and it's more like a brick wall than a plateau, there are a few things you can try. But the prescription depends on what you have been doing to get to this point. Basically, a 'brick wall' is telling you that for the time being, you've gotten all you can out of the approach you are currently following. You need a change.
If you have been only learning a lot of songs and havenít spent any real time honing your skills in other ways, you need more challenge. Apparently the songs (or the overall style of music you are into) haven't been pushing you enough. Check out something like the skill-intensive exercises in Speed Mechanics for Lead Guitar.
On the other hand, if you have been heavy into "exercise-type" practicing, always trying to develop and hone your technique and build your speed -- to you I say, lighten up! Learn some music that you dig. Distract yourself a bit from the whole routine of woodshedding. Yes, you want to improve, but thereís also this little thing, you might remember, something called...MUSIC!? Learn some songs. Jam with some friends, or form a band and rehearse. Set up a date and play out. Write some songs. Record some songs. Isnít that the whole point, anyway? Sure, some conscious dissection of skills (in the form of exercises) is a good thing. But too much of it will burn you out before youíve mastered the instrument. You, my friend, are getting out of balance! (See 2. Balance vs. burn-out.) It's time to forget about those exercises for a while.
BTW, when you play music, youíre not thinking about improving this or that skill. You are taken over by the music. Thatís known as being in the "flow." Thatís the Zen state of "mindlessness." Being completely absorbed in the task at hand. There are no questions of "is this right?" or "will I get that part?" or "why canít I play this faster?". Playing music naturally produces this state of mindless absorption, if only you allow it. All you have to do is get out of the way and let it happen!
Also, a little faith is in order here. Remember that watching your own improvement is a lot like watching an hour hand on a clock. You canít see an hour hand move if you just sit there and stare at it. But what if you got busy with something you enjoyed, then looked back at it a few hours later? Certainly, youíll see that the hour hand had in fact moved, quite noticeably. Practicing exercises with a constant eye toward your own technique improvement is, in effect, like watching the hour hand of a clock. Maybe you were fortunate to see some huge leaps in your technique before. That's not always how things go, though. Sometimes the road is slow. Keep a balanced outlook, keep perservering, and you'll eventually get there.
If you are really burned out and discouraged. The best antidote is some time off. Just donít play for a while. Then go see a few concerts. Let the inspiration return. Then start learning some music again. That should bring it all back.
When you do come back to it, let me give you one other piece of advice: Remember when you first began with those exerices in Speed Mechanics, and everything was new? You were developing new playing habits and improvement was rapid. But after playing a given set of exercises for a while -- any set of exercises -- something begins to happen. Each exercise starts to sink into your subconscious mind (or 'muscular memory') to the point that you are no longer thinking about it. It has become a habit. And at this point, it is no longer going to help you much. All exercises have a built-in 'life-expectancy.' Check out the whole section called the "Art of Practicing" in Speed Mechanics, and start making up your own new exercises. Also, check out the "Improvisational Speed Exercises" I laid out in section 7, Building Speed: The heart of the matter.
A plateau means that you've gotten all you can out of the approach you are currently following. You need a change. If you've been doing a lot of song playing and no exercise-type practice, get a copy of Speed Mechanics and get to work. If your plateau is due to too much exercise-type practicing, you need to get back to music and re-discover your inspirations. Itís not just about technique improvement, you know. See 1. The key to success and 2. Balance vs. burn-out. Also, let up on the exercises you've been hammering on, and focus on the "Art of Practicing" section of Speed Mechanics to begin creating your own new exercise variations.