Much of what I work on with my students during lessons comes back to teaching them how to practice. In fact, practicing well is one of the most valuable things a teacher can impart on their students. It's also one of the reasons taking lessons is so important in the first place, along with learning good technique, troubleshooting things you're stuck on, and being guided on what material to study. Well, I'll discuss the benefits of studying with a teacher some other time. For now, let's talk about practicing.
When I was kid starting out on guitar, I used to hate the idea of what my teachers (and anyone else who knew what they were talking about) would tell me was the right way to practice. They would say things like “don't rush”, or “you have to learn the basics first”. When you're 13 years old and all you want to do is shred like Steve Vai, it's hard to make sense of anything that doesn't involve clicking the metronome up a notch every couple of minutes, no matter how sloppy it starts to sound. Oh well, at least I was using a metronome.
Several years, and some good music teachers later, I began to fill in some of the holes in my practicing, and started getting the results I was looking for. Like everything else I discuss here, this is a lifelong project, but if you're not doing these things now, and you start doing a few of them, you'll see results right away.
Pick apart a single measure; or a few notes/chords. The next time you learn a song, or a solo, take the first few notes or chords, and check to see if you really know them. Play them slowly, and make sure you can repeatedly play them perfectly and effortlessly before moving on. Take the next few notes or chords and do the same. If it is a chord section and the chords are new or challenging, this would involve playing only two chords until they're easy, and then add in the next chord.
No matter how you group your notes, don't forget to overlap parts. Rather than practicing one measure by itself, then the next measure by itself, overlap them by a note. For example, if you're dealing with one measure at a time, practice each measure including the downbeat of the next measure. If you don't do this, after you learn two parts, you'll have to spend extra time putting them together.
Another approach would be what's sometimes called the “chain link method”. Essentially you play two notes, then add a note, one at a time, until a measure/line/phrase is complete. Of course you only add notes after you're comfortable playing the current group. This method takes care of the overlapping for you, but just make sure you start the next section with the last note of the previous group.
Listen...really listen. Listen to what you're playing and ask yourself some questions. Are all the notes in this chord clear? Am I playing articulately? Is the rhythm exactly right? Am I rushing/lagging? Is the balance between bass and treble right? What about the guitar tone; is it appropriate for this song/genre/mood? How is the guitar volume relative to the recording or the rest of the players in the room? Be honest about the answers. Don't settle. Always try to improve your listening and come up with solutions and exercises to solve the problems.
If you're making mistakes you're going too fast, or playing too many notes. Don't ever practice mistakes. Of course you're allowed to make a mistake here or there, but as soon as you're making several mistakes in a part, or repeating the same mistake, it's time to slow down, or shorten the length of the section you're working on. Most of the time, my students need to cut down their tempo by something much more dramatic than they think they do while they're learning something (until they get really good at practicing, that is). If you're always practicing error-free, you'll start to see a huge improvement in the time it takes to learn something new.
To sum it all up. Basically it all comes down to what I used to hate hearing as a kid. Take your time, don't rush, do it little by little, pay attention, etc... But this stuff is really important, and will get you the results we're all looking for: learning songs sooner and more accurately, playing cleaner, easier time learning tough parts, and making steady progress without checking the clock every five minutes. Good luck, and have fun practicing!
I know I didn't cover nearly everything here, so look for parts “Practicing, part 2,3,4...” in the future!