I hope whoever is reading this post doesn't expect to learn how to play a guitar solo by reading this post, despite the misleading title. You can't learn to solo from spending five minutes reading some random guy's babbling. But I will tell you some things that may perhaps help you to think of playing solos in ways beyond those you may consider currently. Ironic side note: I'm hardly someone who would be considered a great lead guitar player. I'm sloppy, and I don't go for playing designed to impress other musicians. That having been said, you still might enjoy some of my thoughts on the topic.
This may seem obvious, but it's important to say. Want to learn to solo? Well, guess what: many guitar players have done many solos before you, and like most things in life, you can draw upon what's been done in the past. Listen to your own favorite solos. What makes them great? What makes you excited to get to the part of the song where the solo resides? As part of the listening process, try playing along with the solos you respect. Even if you can't hit every note at the tempo you'd like, play along, and learn the patterns of fingerboard motions for that player. It's a great exercise in discovering why the original player chose the notes that he or she did.
2. Pay attention to details
Ever try and learn someone else's solo note for note, perhaps through a tablature site, and yet it still doesn't sound right when you play it? Guess what: it goes beyond the notes, and beyond the tone. How you approach every single note, how long each note is sustained, whether or not there's vibrato on the note, and how you move toward the next note... these are all fine details that will help you be a better creator of your own solos, when you get to that point.
3. Ask yourself, "Is this solo even necessary?"
Let's take a little walk back through time. Before the advent of rock and roll (which we'll get to shortly), there were indeed solos. However, they were a little different. A jazz trio in the '50s would perform the song's basic melody and chord progression, and then each member would have a little solo section. These solos were played over the same progression as the tune. As rock took over and the guitar became a predominant instrument in popular music, more and more often, the solo section would take up a bigger portion of the tune. This probably peaked in the '70s and '80s as hard rock/metal, progressive rock, and "guitar shred" music became mainstream.
But then something interesting happened. With the advent of alternative music in the '90s, the solo became less of an essential part of a song. In fact, heh heh, the inclusion of solos meant that you were living in the past. Today, music is a little more open to whatever the writer feels is good for the song. Solos are neither expected nor verboten. But still, don't put a solo on a song because you think you have to. Do it when it's good for the song, and only then.
This weekend, I worked on a solo for the in-progess song "Picked Up off The Floor" by my band They Stole My Crayon, which you can listen to above. It's not done yet; I certainly didn't do a perfect performance on the first take, seen here. But I am happy with the approach I've take to create the solo, which should come out great when it's really ready.
4. Then ask, "Is this solo adding anything to the song?"
What makes a solo good for a song? Perhaps you want something to break up the monotony of your verses. Perhaps there's a bridge in the song that doesn't seem to require lyrics, but needs some sonic interest beyond the basic chord structure. The point, once again, is that the solo should benefit the tune. Try and stay objective, and ask yourself if it's really bringing something interesting, or just unnecessarily extending the song's length for no purpose. Have the wherewithal to abandon that solo, if it's not needed.
5. A tune within a tune
The most simple guitar solo is just the melody of the song done on guitar rather than voice. Kurt Cobain's now-classic solo on "Smells Like Teen Spirit" is just that. He plays the same line he'd been singing in the verses, and guess what? It works. Other solos are different. They may draw from the song's melody, but add various elements and flourishes that are new. And some solos -- especially those in a song's bridge, which might be in an entirely different key than the rest of the tune -- are songs of their own. For those, you need to go into a different mode of songwriting, where you're creating a new little song within your song. Some of the most memorable solos are like those.
6. Can you sing it?
One test of a solo that people will appreciate is whether or not you can sing it. Why would you want to do that? Well, if you can hum around with the solo, that means you've given it enough melodic interest to stand out on its own. Some of the more well-known melodic soloists include people like Brian May of Queen, Elliott Easton of the Cars, David Gilmour of Pink Floyd, and many others whose solos add so much to their songs that often people regard them as being more integral to the song than even their singer's lead vocals. Can you sing along with Jimmy Page's epic solo toward the end of "Stairway to Heaven", or Jay Graydon's mind-blowing solo on Steely Dan's "Peg"? Of course you can. And that's why they're some of the greats.
7. Speed kills
Look, we're all impressed by fast playing, to a degree. But speed without melody is the kiss of death for most guitar solos. I'm not saying that you can't play speedy lines in a solo! But I am saying that fast playing tends to kill melody, and generally only is of interest to other guitar players (and often not even them). I understand that when you first learn how to do an arpeggio sweep, you want to add that to your repertoire. Fine! But don't do it throughout an entire solo, please. Variety is the spice of life, and the secret to a cool solo.
8. Don't fear simplicity
If you're of the opinion that good music offers a feel and a vibe, then a solo doesn't need to be complex to be great. Neil Young's solo on "Cinnamon Girl", for all intents and purposes, is one note. Why does that D note played over and over again seem perfect for that song? I don't know, but it does, and hats off to Old Neil for having the balls to do it.
9. Learn scales, but for God's sake, don't use them literally
Gaining an intimate knowledge of scales is crucial to good and interesting solos. If you're stuck in the same pentatonic blues scale on every solo, chances are high that you're not an exciting soloist (unless you are simply amazing at stretching tons of vibe and feel into each note, and let's face it; most players aren't). So, expand your chops and get into different ways to use modes and minor scales to make your solos super melodic. That having been said, there's nothing more cringe-worthy than a solo that sounds like someone practicing a scale. A little tip: good melodic playing tends to have both upward and downward pitch motion, and rarely uses more than a few consecutive notes in a scale before skipping around a bit. Also, vary your timing. A bunch of consecutive eighth notes are fine when you're practicing, but rarely do they make your listener excited in the context of your song.
10. Practice (duh)
The last thing I'll tell you about solos is the part you probably don't want to hear. Everyone wants a quick fix. Everyone wants immediate results. Guess what? Learning to play really cool and interesting solos doesn't happen overnight for anyone. People who you probably consider masters of the art of guitar are often those who are still aware that they can improve in their playing, and continue to practice accordingly. Pull out your metronome and start playing scales, paying attention to your picking technique, how well you fret each note, and all of that. You will get back what you put in.