The two biggest things you can do are:
1. Make a commitment to finish.
2. Don't be afraid to discard, rework, or combine.
I usually have about 5 songs going at once. That way, I'm always inspired to work on at least one of them. However, if I start wanting to begin new songs, I force myself to finish an older one. By finish I mean record lyrics, mix properly, and master. AT some point you have to tell yourself: I'm going to finish 2 songs this week, and do it.
A great way to do that if you have a bunch of so-so songs is to combine them. Take the good bits out of song A and put them in song B. It may be that the chorus of song B fits the verse of song A well (after you transpose it). If so, do it! Don't be afraid! Some people are timid to trash their own work. Don't be! Through destruction comes progress.
I would say on average each of my songs starts as three or four sketches. I've had people tell me I am good at melodies. The way I do it is to make 10 or 20 melodies, and then throw 9 or 19 of them away. The one I keep is a good one.
I find that much more effective then continually polishing. Make more new stuff, then combine!