I know this is a cliché answer - but I let the song dictate if the musical composition needs to be more complex. Most of what I write are based around lyrics that tell a relatively relatable story - and often simple is best. I try to be a little more creative than a standard I-IV-V - but many of my songs have 7 or less total chords (I try to make the arrangement clever if the chord structure is basic).
That being said - I have forced myself to stretch when composing a few instrument pieces where I try to write more complex chord structures and arrangements and try to creating complex orchestral arrangements - which often pushes my 32 channels of MIDI sound modules to the max.
On occasion, I'll bring in someone who can execute a performance better than I can - but for the most part, I simply push myself to learn and execute the parts that I need to complete a song. I've probably improved my playing more by forcing myself to learn parts than I would by practicing scales, etc.
