The short answer is you just play them.
But if you’re reading this the question in your head is more like, “How do I practice guitar scales to make them sound good?” And that’s where it gets tricky. What is good? Melodic is good–that is, something catchy that you could hum or whistle. You can’t hum a tapping riff.
For others good is SHREDDING! Breaking the speed barrier, running all sorts of flashy moves up and down the neck. Not always melodic and not everyone’s cup of tea, but hey, it’s FUN!
Scales tend to get a bad rap. Aside from their difficulty you have school bands using them merely as warmups before working on songs for their next concerts. Can’t blame the teachers, as they don’t have time to develop the improvisational skills of thirty kids in a couple hours a week. So I go to teach a major scale to these kids taking a break from concert band and this look of horror creeps into their eyes…HOMEWORK!
Then I do the same scale with lots of distortion the way Satriani might do it and they go, “I want to learn that!”
Scales are like an alphabet to a new language. Playing them back and forth is important to learn how the letters work, but the communication of music is found in how you string those letters into words and sentences. It takes time, you’ll hit the occasional plateau, but in time things will click and your musical intuition will grow. How to make it grow faster? Hmm…
For starters, learn one scale in one octave or maybe one position (that is, one area on the fretboard) max. Want to learn the entire scale on the neck right away? Go for it–but only if the inspiration takes you there. Personally, I lean towards learning how to make one scale sound amazing instead of just memorizing all the shapes at once.
It will probably sound mechanical for awhile. This doesn’t mean your playing sucks, it means you’re building the muscle memory of the scale into your fingers so later you can focus on the music instead of the mechanics. Which reminds me…try to use the same fingers for each note of the scale EVERY time you play it. You walk the same way every time you move, right? Or do you try two left-steps then a right and other combinations to see if you can walk faster? Didn’t think so!
Playing the scale back and forth will probably get stale fairly quick, which is why you’ll need to learn sequences, which are patterns within the scale that shuffle the order of the notes in an organized way. Google the subject or ask your teacher to get started. I detail a few patterns in my pentatonic blog, linked below. These will sound mechanical at first as well, but with time they will start to make sense in a musical context.
Practicing with jam tracks is a great way to hear the scale used against a harmonic backdrop. I’ve got a few such jams here. Your soloing might not sound musical yet, but you’re training your ear to hear the scale against a rhythm section, which will develop your rhythmic and melodic sensibilities.
There is a massive list of techniques you can practice within a scale and I’m sure you’ve heard of many of them: Hammer ons, pull offs, string bends, tapping, string skipping, etc. You can Google or Youtube any of these and find material to practice. Each concept you absorb into your scale will make it that much more interesting to play and listen to.
Once you’ve learned the multiple box shapes of a scale in a particular key you should go beyond the box shapes by learning the entire scale up and down EACH STRING. The goal is to see the scale across the entire neck, not just in zones.
I like the question & answer technique in bringing a little structure to your riffs. To try this concept, pick a five note pattern in your scale–that’s your question. Now play the same pattern except change the last two notes–the answer. Sort of the same riff, but the different endings make a musical statement. From there you can expand the idea into as many notes as you’d like.
I came across the “pivot riff” technique awhile back. Pick a note in one position/box shape of your scale–that’s the pivot note. Pick it twice. Then pick another note in the same position twice. Then come back to the pivot note twice. The long term goal is to hit any note in the box shape and alternate it with the pivot note. It challenges your brain to work the scale in other ways than the first way you learned it, back and forth. Try different rhythms to keep it fresh. What–you have no ideas? Perhaps my guitar chord poster can help, as it also has rhythm exercises and strumming patterns. 
Another exercise I like takes a bit of willpower and patience. Solo on one note for 5-10 minutes. That’s it. How expressive can you be with one single note? If you give it time, you’ll find there are a lot of ways, from how hard you pick to how long you let it sustain between plucks. Or what what if you added silent pauses between the notes? Use a jam track to help with the inspiration.
The big thing in scale development (or anything about guitar, for that matter) is TIME. That doesn’t mean you need to play one scale for hours a day. A concentrated five minutes three or more days a week will get you cool results. Obviously if you want to play like the Guitar Gods you’ll need to invest more than that, but maybe you just want your scales to sound more musical and aren’t concerned with playing like that. So don’t stress over it! But invest that five minutes a day and you’ll realize the payoff sooner than later.
All of these concepts take TIME! The point of this blog is to give you an idea of the concepts you should pursue in making your scales sound musical. Specifics are all over the place, online, private teachers, books, DVDs. Tackle one idea at a time instead of trying to do them all at once, which might go in the “duh” file, but I still get students wanting me to show them every technique in their first lesson.
You can’t learn a language in a day.
For extra blog reading I’ve written a few other related articles…
How the heck do I write a guitar solo?
Guitar & Bass T-shirts now available!
great post and advice. i dig the blog. i added it to my google reader. keep it up.
A good tool for guitar scales on the iPhone is iPractice:
http://reviews.cnet.com/8301-19512_7-10344761-233.html
Great post – I’ll definitely try some of these techniques.