The ball goes to your forehand, you lean that way, and . . . suddenly you can't move. And so you lean more, and perhaps you are able to making a flailing, off-balance return. Then you watch a top player move to the ball, and while in perfect position, he makes a perfect forehand. What goes through your head? "He has a better forehand than me."
What's wrong with this picture?
This type of leaning footwork is pervasive at the beginning and intermediate levels. It happens on regular drives and loops, and even more when blocking, where players reach instead of stepping toward the ball, and wonder why their off-balanced blocks are so erratic. It's also a primary reason why so many players are unable to play from off the table. They are so used to reaching for the ball when closer to the table that when they are off the table, and have more ground to cover (but more time to do so), they fall into their old habits and lean in the direction they need to move, thereby making it impossible to actually move in that direction.
If you need to move to the right, step first with the right foot. If you start by leaning to the right, then your weight is on the right foot, and you can't move it. Similarly, if you need to move to the left, step first with the left foot. If you need to move in, step in with either foot (the right foot for righties against short balls to the forehand, which are usually the tough ones to reach), but whatever you do don't just reach for the short ball (which means leaning) and then wonder why you can't reach it or are off balance if you do.
There are also two ways of taking that first step when moving right or left. You can start with a short step and then shuffle both feet together, or (especially when moving to the forehand) you can start with one long step. The short step and shuffle (called "two-step footwork") is considered the norm and is the most taught method, but many or most top players start by taking one long step. (This was pointed out and shown on tape at the ITTF coaching seminar held in Colorado Springs in September, 2010.)
How do you learn to step instead of lean? Practice. But not just at the table - you can practice anywhere. Make a habit of shadow practicing your footwork - it's great practice and exercise, and you don't need a racket to do so. (But if you want to have a "racket," almost anything will do - I've been known to shadow practice with a stapler.) So leave behind the leaning and get in step by stepping.