1) Try a new pick. Yes, a pick can change your tone. Try thicker or thinner as well as different materials. Rumor has it that Billy Gibbons got his tone using a Mexican Peso as a pick.
2) Change string gauge/type. Coated v. uncoated, and different materials can quickly change brightness, punch and related tonal properties. Changing gauge can add thickness or bottom end to your tone as well. Remember, changing gauge requires neck AND intonation adjustments.
3) Vary your pickup height. Back your pickup down even a turn or two and you can add clarity while slightly reducing output, which changes how your amps sounds as well. Bringing your pickup up can add output and gain, but watch out for feedback if you get the pickup too close.
4) Re-dial your amp! Turning the knobs is a great way to change tone of course! Remember, the guitar is a predominately mid-range instrument. Suck out the mids and you suck out the key frequencies of your instrument. Don't "dial with your eyes." The numbers on the dials are kinda arbitrary. Not all amps are alike and just because you think it should be dialed a certain way doesn't mean it should. Mids are critical, gain is best when rolled back for a thicker sound and as important, understand the difference between presence and treble! Remember - vary the volume and gain to get different tones. See my other post on the relationship between volume and gain here:
5) Eliminate all your effects. Most pedal boards or rack systems will eat tone also if they're not set up properly. So take them out for a while and get all your tone back. Then start to add back in only what you NEED.
6) Double your tone. Use your delay pedal and just add a quick, single repeat, slap-back delay. It creates the effect of two guitars. Don't put this in your loop. Drop it in front. The delay time should be something very short. You're not trying to hear the echo so much as create the feel of two guitar players playing at the same time.
7) Change the volume pot in your guitar. Varying values can wildly change the way your guitar sounds. Many hum/hum guitars come standard with a 250K pot. Using a 500K can create a very different feel. If you play with EMG's, try chaining the 25K pot for a 250K and get a warmer, more organic sound.
8) Vary your pick technique and fretting. Holding the pick too loose will create a weaker sound and too tight your tone may get too harsh. Keep in mind that good picking should have the pick crossing the strings at a bit of an angle. Vary it. If your pick hits the string too flat, not so bueno. On fretting, remember that you should fret just behind the fret, as close as possible but not on the fret. If you're on the fret, you'll mute or dull your tone. Vary these things for optimal results.
9) Check your amp's bias. If it's too low, your amp will sound cold, dull and lifeless. If it's too hot, the amp will sound bright and harsh. Oh, and you're tubes will fail faster. Check with your amp's manufacturer for appropriate bias. Enlist the help of a qualified tech if you don't know what you're doing. Tubes change over time, so bias should be frequently checked!!!
10) Get a new cable! Cheap cables (or cables that are too long) can eat tone. And remember, fancy jackets don't mean high quality. The guts of the cable matters. Cheap cables always have lame guts. Try a high quality Mogami or a 4 conductor Beldon Brilliant from CBI. These can add juice back in. It's kinda like towing a heavy gear trailer with a VW bug versus a GMC 8 cylinder truck. Ok, this one may not be under $10. So I'll give ya a freebie. . .
11) Practice! Remember, most tone is in your fingers. . .