Well, I am a singer songwriter that makes money off of my skill. I can't claim wild success just yet, but I haven't been actively touring in ten years and have only recently started back up and have begun to do it for a living again. I can give some advice that you can take or leave if you want to do this as a living.
Be Honest With Yourself
Most songwriters suck. Really. Most people can't master the skill set required. Honestly compare your work to the work of those that inspire(d) you. Are you at or near that level? Can you get there? If not stop now and invest your money in a business degree. Do people that you don't know and have no connection to you constantly tell you that your songs are great? Do people come up wanting to buy things from you at gigs (like CD's and T-shirts) without being told that you have those items? Do other songwriters who you respect like to listen to your songs? Are they bullshitting you? Do people seek you out to perform?
Sit down and honestly assess whether or not you have the goods not just based on your own belief in yourself but in objective criteria.
Pick A Genre That You Are Good At
If people love your stuff and ask you back and throw money at you because you write country drinking songs and you decide to write pop songs expect different results. This isn't to say that you can't be versatile or blur lines --you can and should-- but do what you are good at and comes easiest to you, not just what you think the market requires. Realize that to do this you may sacrifice your shot at the big time by staying in the more obscure genre's (jazz, blues, folk, instrumental, etc...)
Throw out your TV
Get rid of it. Start reading. The TV requires passive participation and is a huge time waster. It rarely inspires or causes one to think and mull things over. Reading, on the other hand, is very fertile grist for the mill because it requires one to actively participate. If you leave the TV on for background noise turn it off and turn on music. Because we actively interpret the emotional reaction to a pieve of music it can often inspire us in a manner that the passive pumping of TV cannot.
Write Every Day
The more you write the easier it is. If you want it to be a job then approach it as a job (just a job that you happen to love.) --You DO love it don't you?-- Write seven days a week about something. Find something. You're not inspired? Get inspired, make inspiration happen. Pick a topic, a subject, a theme, and then write about it. Don't know enough or aren't feeling it? Go read more about it. Look at art and photographs about the subject. These won't all be good, but don't throw 'em away. Keep 'em all and review 'em. You'll find that some that you thought were great when you wrote them positively suck a few months later and some that sucked can be salvaged. Don't get married to the song, look for lines, phrases or ideas from bad songs that are diamonds in the rough.
Forget Inspiration
Jobs aren't supposed to be easy. If you only write when you are inspired then you will have a pretty thin songbook. Either you can write songs or you can't. If you can then do, even when you aren't white hot and burning as you scribble lines furiously across the page. Inspiration is nice when it happens but it is still temporary. You are going to have to make decisions about songs that you were inspired to write that you aren't inspired to arrange, so why not vice versa? If you're a writer then write.
Practice, practice, practice
Your instrument. You don't play one? Why not? OK. You can become a successful songwriter by not playing anything as long as you team up with someone, but this is a business of razor thin margins down here in the trenches why are you giving half of it away instead of learning the seven to nine chords that it would take to rough in almost any song?
Play in front of people
If you already play an instrument get out there and start playing in front of people. You can't sing? So what? Neither can Bob Dylan. Unless you are attrocious and can't stay on pitch then get out there. If you are attrocious then find a side man (or woman) to handle vocals. The point is to get out there and get honest feedback about how good your songs are and to supplement your income. If you live in a music center city then this alone may generate enough buzz over time for you to transition from a weekend musician to a working one. At the very least it allows you to start networking with other musicians and songsmith's who are more inclined to give you honest feedback than the drunk guy in the Dead Kennedy's T-Shirt.
Don't Give Up
If you honestly believe that you have what it takes and that belief has been substantiated then don't give up. Committ. Start actively persuing your dream of working in the music business by working at the music business, gradually fade out all non music jobs while fading in all music jobs. Be diverse. Apply for government grants, look at arts councils, enter songwriting competitions, attend and play in festivals, work at radio stations if you can, do roadie work if you can get it. At some point you will reach a breaking point where you must decide to jump in and give it a go or remain an amateur. If you have what it takes jump in with both feet, cut the ties to the day job and start selling yourself ferociously. Gig, write, sell, create then gig some more. Play everyplace they won't arrest you for it, talk to everyone who might remotely be able to help you, then do it all over again.
Believe in Yourself
I believe I am a fantastic songwriter with unlimited potential given the right stars alligning. There is a possibility that I will labor in obscurity for 35 years while doing nothing more than keeping the kids fed, the mortgage paid, and the car running. I'm OK with that. It will not affect my belief that I am a fantastic songwriter. If you don't think you are great deep down in your soul then get great. If you are great, don't ever stop believing it whether or not you are selling 50,000 CD's a week at Wal-Mart or 50 a week out of the trunk of your car.
Forget Getting An Agent
Tom May of NPR's River City Folk told me that for most newcomers and obscure genre artists an agent will not even sniff you unless you are making in excess of $200 a gig and are gigging twenty times a month. It just isn't worth it to them. It's not worth it to you unless you are pulling in $2,000 a gig and playing 10 gigs a month. Are you making 20K a month? No? Then start handling your own press, promotion, and booking. You'll get more work and more money that way. Get a press kit together, start doing your research, and start aggresively selling yourself.
That's all I got for now.
to keep the thread short, but all that is really great advice. My one addition: practice singing. Many people don't, many believe you're good or you're not. Practice practice practice. It helps tons.