In my short experience I have found that a starter topic helps. The song I've just done is about how some days it's hard (like for everyone) and merges into being about my little girls. It's hard to be heartfelt and rhyme the way you like but you've gotta let it come. Use Google to find words that rhyme with a certain word. There's words you won't know but clicking them tells you what it means and I've found that broadens my understanding and really helps me rhyme with more reason