There are a lot of great tips and techniques here. I would add that it can be very helpful to try something new.
Many times we fall into a rut writing the same type of lyrics with the same song structure, rhyming schemes, thematic ideas, etc. Sometimes, doing something different, even if you decide not to pursue it later, can be break down some walls. Feel like your lyrics are too literal? Try writing some poetic free verse, focusing on imagery instead of story. Limited vocabulary? Read more from authors/lyricists you enjoy and examine their word choices. Bored with predictable rhyming schemes? Put the music aside and just write lyrics without considering the flow of the music.
I've talked to lyricists who struggled with trying to write complex concepts and themes. When we feel passionately about an idea or a conviction, it can be very difficult to express it without sounding preachy or using cliches. I suggested trying to express only one idea or concept per song. Don't give me an opus that attempts to explain all of time and space in a single song. Just give me one nugget of truth for this song. You can give me more in the next one. One of the traps lyricists fall into is trying to be Profound. You know...it's when you go so overboard in trying to be Deep and Meaningful in your lyrics that you come off as Stupid and Lame instead? Some of the most meaningful lyrics of the last few decades have been simple statements.
One of the harder pieces of advice is this. Try reading the lyrics critically. You know how people sometimes mean different things when they ask what you think? Some want you to find all of the things you can praise about their work. Some want you to find anything you can criticize. If you can't bear to hear the negative, then you aren't ready. If you seek out good criticism, you're ready to refine your lyrics and improve as a writer. It's not as easy as it sounds.
As writers, we go through stages where we need encouragement to try to reach our potential. We also go through stages where we need someone to rip out the garbage before we can actually get there.