Quality Control Exercise
Many times I hear students that can play all kinds of fancy stuff, but it doesn't sound good. And it's important to remember that most listeners couldn't care less about how technical something is, they just care about what it sounds like and how it makes them feel!
In my early teens, I added a 5-minute Quality Control session into my practice routine and it really helped my tone. I also use it when I get a new instrument and I want to really focus on getting the most out of it - it's one of those big deal exercises once you get into it. Lotta depth.
How to do it
Play a note (any note) and really concentrate on the sound, the tone, the texture of it and make it as great as it can be! That's all there is to it - just really focus on the quality of the note you play. Then play another and another, maybe a couple together, or a bend - whatever - just focus on the quality of the sound.
You can use it for chords too but better to start with working on individual notes.
Watch out for
How you place your finger on the string makes a big difference to the tone, I first heard of this in Eric Johnson's awesome VHS (ancient type of video device!) Total Electric Guitar where he talks at length about what he calls "Positive Finger Placement" and that's what you want to be looking for. There's no right way for everyone approach - you have to find your own!
How and where your pick touches the string also impacts on tone. Experiment, see what sounds best to make a sound that YOU like and you'll be on the way to creating YOUR sound (if that's what you want!). Exploring this will also help you widen your available tonal ideas and widen your expressive tools - with the added bonus to helping you imitate licks or sounds you hear that you'd like to copy!
I recommend just 5 minutes a day, 5 days a week. Amazing what can happen when you really focus on something even for short periods of time. Eventually, you want to develop a little brain 'sub-routine' that works on this automatically all the time, so you're always aware of the tone of what you play - but that's likely to take many years to become automated.
Do it for weeks, months or years - it's the kind of exercise I still keep in my Pick'n'Mix Box and do from time to time because it's got a whole lotta goodness in it :)
Hope you dig it!