Things I do:
1) Don't be a critic until after a song is written. During writing, it's whatever comes out. If nothing comes out, move to another part of the studio work.
2) Other part can be... grabbing an older song and remixing it with my newer experience and toys. Learn to be the engineer.
3) Other part can be... making or grabbing a quick demo and listen to it in the car. See what needs fixing over a few days on the way to work and back.
Some times it's a poor work in the lyric. Sometimes its a poorly sung part, how do I sing it better (change how I form the sound, change the word, change the melody to an easier part of my range) Sometimes it's a bad chord choice for the story-line. See what other chord changes can work there (sometimes it's just a variation of nearby chord)
4) To build interest I make sure the LAST thing I do before sleeping is to listen to music or play music. Then my mind mulls over it during the night. Keep a recorder handy in case you get some motiffs of ditty's.
5) During the day, work on something hard. Like how to harmonize, transpose, remember certain chord sequences, visualize the circle of 4th/5ths. Your mind likes problems to solve. Compared to theory, making music is more fun. But if you aren't making music, this theory learning part can give you tools to fix problems in your songs with a better understanding of music structure.
6) Talk to your Muse. That one "fan" that listens to your music or says good things about it. Anticipate how THEY will want to hear your next work.
7) Taking a break isn't a bad thing. Sometimes you need a break. Mine sometimes go for 6 months! But this isn't my profession. You can do that for a hobby.
.Play your instrument when you otherwise would "snack". This way you avoid calories and get some chops out of it. Sometimes you come up with a dirge that can become a song!
9) Play some strange drum loops and see what motifs you come up with on the guitar. Some of these can be turned into a future song. Great songs have awesomely simple but rockin' motifs.
10) Ask someone for their poem and tell them you'll try to set it to music. Working on someone else's words makes for some interesting music. Maybe not your best work, but it gets the juices flowing. IF you have written your own poetry, look for ones with good iambic pentameter and squish it into a song. As the programmers say: Reuse, reuse, reuse!
11) Take away all pressure. Your next song doesn't have to be better than the past. It doesn't have to be done on time. It doesn't even have to be a song in the traditional sense. No chorus? Dylan did verse 1, verse 2 all the time. Snippets can be stitched together. A snippet is a great thing to write.
12) Organize your songs. Put them into labelled folders. If you have labelled folders, digitalize them and put them on dropbox. That is, not writing? Then organize the studio, learn some more engineering skills, fix what's wrong with your studio. Maybe you need to finally do that acoustic treatment you've been putting off forever? haha
13) Listen to new stuff and clean your studio. Label your cables while hearing music others told you they like. Even if you don't like it. Work to it. Your brain will soak up musicality