just do it, and don't worry about the lyrics being great. I'm sure if we read the lyrics of most of our favorite songs without ever hearing the music, we'd think 90% of them were pretty stupid.
That said, maybe try to draw upon lyrics from other styles than what you are writing in. If you are writing hard rock songs, maybe suck it up and see what Elton John's (or more properly, Bernie's) lyrics say, how they are constructed, etc. You might be surprised to find that, the music aside, there is some good lyric writing there. Maybe you can pick up some things. Look at some folk tunes - Dylan, etc. Hell, maybe even, listen to some crappy pop or country. What makes them catchy?
Also, don't force yourself to write in strict rhyme or strict rhythm/number of sylables. (I still do this all the time, mind you). But if you start paying attention to it, you might find that a lot of songs with cool lyrical flow are those that squeeze a few extra sylables into a line here or there, or don't get too hung up on strict rhyme scheme. To do this, I find it helps to write your words apart from or even before you figure out the music. Just write a poem. When it's done, make it fit the song (which still may include some rewriting).