Keep it short:  The average length of a pop song is about 3m:30s.  Don't outstay your welcome.  Listeners will become bored very quickly if they think they’ve heard it all before and you don’t give them aural reasons why they should listen to the end.  Above all, don't indulge in protracted guitar or other solos - you might think they're really cool but the listener won't hear most of it - they'll have switched off.

Let a song breathe; a common beginner's problem is trying to fill all the spaces up.  Great hooks can come from the contrast between sections and the sparse or even empty spaces between.  So don’t string things out once the song has done its purpose: if the listener likes the song they’ll play it again and that’s the whole point.

Always be prepared:   Use your mobile phone to jot down or record quick ideas for lyrics and melodies.  Inspiration for that next great song won't come into your head spontaneously.  Sessions where you decide to sit down tonight with your guitar and write a great song always end in disappointment and the feeling you'll never manage it.  Ever had a superb thought for a lyric phrase in the car only to completely forget it when you got home?  Great writers, and comedians, know this. That's why most of them jot down things they hear and see during the day that amuses them, hoarding it for later use. See the quote elsewhere in this site by Richard Thompson: “You overhear a conversation on the bus and think ‘that’s a great first line’.”  

Benny Hill, the famous comedian, always carried a large notebook with him and he'd often sit for hours in restaurants and pavement cafes watching and listening to people. He'd jot down the everyday funny or odd things people said or did and go through them later when he was writing shows, so that he had something to spark his inspiration rather then having to sit down with a pile of blank paper and nothing else. Some writers are inspired by the titles used in magazine articles. These are often snappy, and can make great starting points for songs.  Get into the habit of seeing at the world as an infinite ideas machine.  As a songwriter all you have to do is to tap into this amazing free resource and translate what you see into songwriting ideas.

I keep such notes and have quite a hoard of couplets, or just phrases, for lyrics.  They can be very useful both for sparking inspiration for a whole song, or just referring to when working on a song and looking for that perfect lyric.  I also keep a notepad and pen by my bed, in case inspiration strikes!  

'Rules are made for the guidance of the wise and the obedience of fools'  Don't forget, we're dealing with art here, not science.  Human creativity is a wonderful but unpredictable thing and sometimes breaking the rules makes magic happen, and if it feels right and sounds right, it is right.  The Beatles were the masters of this during their psychedelic creative period - deliberately breaking the rules to see what sparks would come off, e.g. stitching two different songs together in 'Day In The Life'.  But learn the rules - your craft - first!