Oh, dear! Where shall we begin? Scale degrees!
Well, you said you felt stuck on the I-IV-V. How many chords are left? Four. What are they?
ii, iii, vi, and viio. These are 3 minor and a diminshed.
I-ii-iii-IV-V-vi-viio = Harmonized Major Scale (Triads)
I-ii-iii-IV-V-vi-vii% (half-diminished) = Harmonized Major Scale (Sevenths)
As someone mentioned with substitutions, here's the most common ways to get them:
Tonic-Parallel Relationship (Key Signatures)
For every I, you can swap a vi (C/a)
For every IV, you can swap a ii (F/d)
For every V, you can swap a iii (G/e)
Inversions (triads, but predominately sevenths; Tonic-Parallel kinda goes here, too)
Every m7 can be inverted into a M6 (Am7/C6, Dm7/F6, Em7/G6)
Every half-diminshed seventh (m7b5) upon inversion will become a minor six (m6)
Am7b5/Cm6, Dm7b5/Fm6, Em7b5/Gm6
Common Tones:
http://www.youtube.com/watch?v=cp0iBMZ6bG4&feature=g-user-c (What other chord will the melody note fit in?)
Circle of Fourths:
http://www.youtube.com/watch?v=ITft3mjiCQY&feature=channel&list=UL (ii7s for V7s)
Secondary Dominants (V7/ii, V7/iii, V7/V, etc.)
Take this progression: C-Am-Dm-G7-C
Using a secondary dominant: C-
A7-Dm-G7-C; C-
A7-D7-G7-C - Two SDs)
Tritone Substitutions: CMaj7-
Eb7-AbMaj9-G7b9-CMaj9 - Two of them. CMaj7-
Eb7-AbMaj9-
Db9-CMaj9 (Three of them)
Alterations: CMaj9-
A7b9-Dm7-
G13b9-C
Diminshed seventh sub: CMaj9-
C#dim7-Dm7-G7-C
(Note: the 7b9 and diminished seventh subs are identical in sonority. A7b9 is A-C#-E-G-Bb. A C# fully diminished seventh has four identical notes.)
Chord over Chord - G13b9 = E/G7. Actually, this isn't meant to be a sub, but to help the performer identify what to play if s/he hasn't gotten into extended and altered chords. The E/G7 means to play the two-note seventh (G7) in your left hand and the E triad in your right hand.
If you were you play every note from G-E: G-B-D-F-Ab-(B)-E, here you have the, root, third (duplicate), fifth, seventh, flatted ninth, and thirteenth. (9 = 2, 11 = 4, 13 = 6)
If the I chord is a 7, 9, 11 (9sus) or 13, look to see what other chords are in there and choose the appropriate scale degree:
CMaj7 = C-E-G + E-G-B; iii/IMaj7
CMaj9 = C-E-G + E-G-B + G-B-D = iii7 or V/IMaj9
Cm7 = C-Eb-G + Eb-G-Bb = III/i7
Cm9 = C-Eb-G-Bb-D = IIIMaj7 or v/i
C9sus = C-F-G-Bb-D = v7/i or bVII6/I
C13 = C-Bb-D-F-A = v9/I, bVIIMaj7/I, ii/I
The key to using substitutions is understanding how the chords function.
I/iii/vi = Tonic Functionality
ii/IV = Subdominant Functionality
V/vii = Dominant Functionality
In C: C-Am-Dm-G7 - I-vi-ii-V
In C: C-Dm-F-G - I-ii-IV-V (My Girl)
In C: C-Am-F-G - I-vi-IV-V (Heart and Soul)
Girl From Ipanema I: Cmaj9-D9-Dm9-G7(add13)-CMaj9 - I-V9/V-ii-V-I
Girl From Ipanema II: Cmaj9-Ab7-FMaj7-G7#5#9-CMaj9 - I-bVI-IV-V-I
Girl From Ipanema III:
Em7-Am7-G7b9-C6-Am9-Am-D9b5-D7-Dm9-Dm7-Db13-Db9-Bb9#11-A7#5b9-G9sus-G13b9 (iii-vi-V-I-vi ... V9/V-V7/V-ii subV/V ... bVII9-V7/ii-V ...)
Girl From Ipanema III used tonic-parallel, secondary dominant, tritones, inversions, and extensions + alterations to come up with its chord sequence which formed to prolongation. (Four chords became 16 excluding the I chords at the beginning and end)
Here's a video explaining what is called Functional Harmony - this is where substitutions (re-harmonization) come from:
http://www.youtube.com/watch?v=YBNIxsyE9a8