I'm 20 years old. I've shot even par off and on and played high school golf. I stop playing golf during the school year and then play again in the summer. By the end of the summer I shoot in the mid 70s consistently.
However, that is not the case in the Spring, when I start warming up.
What I know starts to help me get back in form is practicing how I would play. Read Tiger's tips (things like picturing a place to land the ball on the green when chipping) and then understand that the more golf you play the more you get a feel for how the ball checks or runs out on the green. Try and feel the distances, etc.
Practicing, like everyone else has said, is key. But practice with a purpose. Understand what you're doing on each swing (DON'T JUST KEEP HITTING BALLS INTO THE RANGE). Take your time.
When you do actually play rounds on the course, play each shot for what it is. Take your time. When I play poorly I find that I'm thinking about what I SHOULD be shooting, not what I actually NEED to be doing. My focus is not where it should be. Play in the moment.
I'm not a golf expert and I won't pretend to be. Lessons are good for technique, but when it comes down to it, much of golf is feel, and that cannot be taught, it has to be learned on your own.
Best of luck to you,
Jim