Here are two things that I find useful in this type of songwriting approach. These are two types of creative ideas that really help make some interesting movements within your songs;

Secondary Dominants
Secondary Dominants are dominant 7th chords that are not part of the key you are playing in, but create a nice movement to a chord that is. You can also use these to change keys in a song. For example, in the key of C; G7 is your V chord. G7 leads you C. Plain and simple. However, say an A7 appears. What context does that have? Seeing as that dominant 7th chords can lead you to either major or minor chords, the A7 can take us to Emi; which is in the key of C. But, we can also go to that A7 chord and have it take us E major. There we have a change in key that sounds natural and intentional. You can also just stick dominant 7th chords in there and have them not lead to where they are supposed to go. This is dependent on you and your taste. However, it is all about intention and confidence. If your ears truly think it sounds good, you will play like that and it will. Try using secondary dominants as a way to change keys.

Diminished 7th chords
Try and stay with me here. Diminished 7th chords consist of 4 notes; the root, b3, b5, and bb7 of a chord. Let's use Gdim.7 as an example. Those notes are:

G, Bb, Db, E

Now within that chord, we have 4 different tri-tones (the 3rd and b7th of a 7th chord.) In this case, a Gdim.7 chord can be used as a substitute for any of these 4 dominant 7th chords;

G and Db = Eb7
Bb (A#) and E = F#7
Db (C#) and G = A7
E and Bb = C7

Now, what keys do these all lead too? Here is how that pans out;

Eb7 = V chord of Ab
F#7 = V chord of B
A7 = V chord of D
C7 = V chord of F

Now you have four keys that all have a common thread. Ab, B, D, and F now have something in common. You will find when you break this down that you have nearly every possible major, minor, and diminished triad available to you. Therefore, you have now opened up your world of possibilities in songwriting.

However you want to approach it, those are a few techniques that hopefully are not too complicated and might work for you. Let me know if you grasp this OK. I know it's a little crazy.

Good luck! :o