Be original!

Always!!!

Some rules I like to follow are: 

• Don't repeat yourself

 If you feel you did something too much (like chord progressions, riffs, solos, etc) stop immediately and start all over again until you have something different.

• Try new rhythms

 You got a cool progression there?, good. Now try playing it with triplets or  mixing some 8th and 16th notes, experiment yourself for a while.

• Silences please!

 Silences are more important than you think, use them wisely and you'll get a nice dynamic. You could create a cool suspense atmosphere and then --- BOOM killer riff came out of nowhere.

• Verse-chorus-verse-chorus-verse-chorus is bad for you

 Let your imagination fly and create intro, bridges, outros, instrumental sections, anything that makes your songs different from any other generic pop song. Music structure is very important.

• Use different scales

Minor scale, major scale and pentatonic are very useful but what about some others like the Spanish scale, chromatic scale, Arabic scale, diminished scale etc. They exist too and you can use them whenever you need. Try different modes like Ionian, Dorian, Phrygian, Lydian to get extra flavor.