Mac's point about being prolific is key. Now - for some, that means that one needs a challenge - something to push them to say "This one is done, gotta do another."

For me, my songwriting became prolific in a self-imposed challenge - the February Album Writing Month challenge, where people worldwide commit to writing 14 songs in the month of February. That's one every other day.

I've had to do quite a bit of studying and learning to keep up. I do know basic music theory and that has been a boon.

Other tips and places to help:

Transcribe (Mac said this already) to understand the 'why' of the melody.
Participate in songwriting challenges like FAWM or the 50/90 project where the challenge is to write 50 songs in 90 days.
Read Daniel Levitin's "This is your brain on music"
Read Matt Blick's blog on Beatles songwriting: Tickets to write, where he analyzes common chord and melody tricks that the various Beatles used in their songwriting: http://beatlessongwriting.blogspot.com/p/tickets-to-write.html
Big interval jumps usually don't work so well in melodies
Make observations about melodies you like and how they interact with the timing of the lyrics, the breaks, etc. If you can't do this, it will be difficult to come up with your own ideas that will actually work.
Don't be afraid to write just for the sake of writing - something good can come out of the end of it.

I've done FAWM for 4 or 5 years, writing almost 14 songs a month in those years. Out of those, I have 6 or 7 quite good songs (by others accounting, not my own).

That's a success rate of about 1/10. Another few years and I'll have an album's worth!

Here's what I did this past year (only recorded 12 of the 14):
http://fawm.org/fawmers/scottlake/

A few of those deserve some further attention, editing, re-writing re-recording and mixing, etc.

But it wraps back around to Mac's point - which I am agreeing with - that being prolific; disciplining oneself to make product, it at the heart of being successful with songwriting (and with anything in life, actually). Commitment. Stick-to-it-iveness. etc. Practice, practice, practice. Malcom Gladwell's "Outliers" should be some inspiration here as well.

On the FAWM site is this quote from Jack London: "You can't wait for inspiration. You have to go after it with a club."

www.fawm.org The best thing that ever happened to my songwriting was deciding to do this 4 or 5 years ago.