This is the real answer to solving songwriting blocks (although the rest of the tips here do help). Writing by sheer force of will is incredibly difficult because we only have a finite amount of it. You can use it in other ways throughout the day and find that you have run out of willpower when you want to write a song. What you really need is the power of why.
Listen to music, analyze a song, write down some interesting song titles or concepts, play with your instrument or read poetry. The ones I've completed end up sounding silly and make me not want to write a song again lol. Since a song is a complicated mix of noises, it's very easy to create a complicated mix of noises that doesn't sound like a song. Bass guitar, cello, a lot of musicians write a lot of good songs for a bass line, it can be done pretty easily if that's your thing.
If I don't realize the songwriting techniques or think, “That thing you just said could be a great idea for a song on a daily basis, I probably don't write very often. Most of my songs are of the form (verse 1 chorus, 2 chorus, bridge, chorus), which is, I think, the typical format of pop songs, and most of the time I realize that, whatever idea I was trying to express, I have already done it properly in the first stanza and the chorus.