The first ting I noticed after I had started actively pursuing an interest in music theory was that many of the bands I aspired to write music like did what I used to think of as "breaking the rules". By this I mean using chords that weren't diatonic to the key they were playing in, outside notes for soloing and such. I now know that there is no such thing in music as "breaking the rules" (not unless you're studying species counterpoint, this is!), but rather artistic choices you make as a musician to add interest & color to your music outside of I IV V progression and its variants.
Perhaps the best & most well known example of this would be The Beatles, you may have you heard of them? Okay, well I'm sure if you're reading this you're already aware of their great impact upon music of all kinds, and love them or loathe them they practically wrote the rule book for pop music for the next half century to come! Well anyway, The Beatles broke much ground with their seamless blend of diatonic & non-diatonic chords in their progressions, and I will attempt to teach you how and when to use them is this lesson.
First off, I will assume that you the reader has at least a basic knowledge of chord/scale construction, but if not this site has many great articles on the topic you would be wise to check out. If you are already up to speed, lets move on!
Minor iv. Perhaps my favorite of all The Beatles songwriting tricks (and the basis for this lesson), subbing a iv for a IV in a major key progression can add some needed interest to an otherwise generic progression, giving off a nice minor key flavor. Another cool thing to do is to play a major IV and immediately follow it with a iv. Very Beatlesque.
Now for some examples:
The Beatles - "In My Life" (Verse)
The Beatles - "When I'm Sixty Four"
One thing to think about when using a minor iv is a progression, though, is that the new chords minor 3rd is going to clash with the major 6th of the key you are playing in, so keep that in mind while improvising or soloing using a progression such as the ones I shown you or one of your own.
I I7 IV iv I
C - C7 - F - Fm - C