First thing's first: Try to come up with a title before anything else. Eventually, this will help your ideas flow more easily, as you try to relate your song to the main title as much as possible.
Once you have figured out the basic idea(s) of your song, make sure you use words that truly relate to its theme. Write down those words as you come up with them. If the title of your song is "Cold Nights", likely words will include "despair", "loneliness", and so on.
The first line of your song should deliver a smooth impression. Remember what they say, there is nothing more valuable than a good first impression. Use punch lines, popular sayings, something controversial, or simply a nice melody that will stun your listeners.
Use unique words and phrases! Do not overuse sentences that will leave people scratching their heads, but DO make sure your song is like no other with phrases that can deliver a powerful message.
Do not make a song too long. Typically, a song lasts about four minutes (average.) The last thing you want is to bore your audience to death. Make sure your verses are short, and focus more on the main chorus that will hook your listeners.
The most memorable songs in history have been about love. Love is something that we all feel at one point or another. Because of this, people are more likely to listen and feel your music. The best ingredient to a song is feeling. Make sure your song has plenty of it.
Listen to a lot of hit songs out there and ask yourself what made these a big hit. Learn from others and apply this knowledge to your song.
Keep a notebook or voice recorder next to your bed. People often come up with powerful ideas when relaxing. Make sure you write these ideas down as soon as they hit! You do not want to wait until the morning, and realize you no longer remember your brilliant ideas.
Practice, practice, practice. When listening to other songs or while sitting quietly, train your mind by improvising about anything that comes to your head. In time, this will make the way you think more flexible, which in turn allow your ideas to flow more easily.
Ask for feedback. Sometimes it is better to stop and ask your friends about your song's progress. If you find yourself in need of some perspective, there is nothing better than allowing people to talk about your writing before making the final version.
As stated before, make sure your song is not too complicated, but nevertheless use good phrases; especially those catchy ones that will keep people coming back!
Write about unique situations that can raise awareness. Whether you write about social issues or a personal, emotional experience, your song needs to stand out from the crowd.
Study what everyone else is writing about. If singing about a certain topic is already starting to frustrate society, this is your chance to come up with a BANG! Write the unexpected. But before you do this, analyze and find out whether your topic could potentially have a big demographic/social impact; you do not want to write a song no one cares about. Simply make sure you know your audience!