A fast, down-the-line serve from the backhand corner (normally done with a forehand pendulum service motion) is a valuable serve. It's great at catching righty forehand loopers off guard as they look to loop your serve from the backhand corner. It's also sometimes good against more neutral players, as they aren't used to fast serves to the forehand and so can be erratic against it. It's even effective against lefties, who are often guarding against serves wide to their forehand, and so are vulnerable to this sudden serve to their wide backhand. Against lefties, however, you'd look to ace them by going crosscourt to their forehands, though you are less likely to get an ace as players generally guard the crosscourt angle. (Lefty servers, this serve is effective for you as well - but I'll let you work out the specifics.) So develop this serve or you will be handicapping yourself against some players.

If you aren't sure how to do the serve, ask a coach or top player to demonstrate. But the key is to fake as if you are serving crosscourt, then at the last second change directions and serve fast topspin down the line. Contact point should be a bit behind the table so that the first bounce can be near your own end-line, which gives the serve the maximum amount of table to drop and hit the other side. (This is true for fast crosscourt serves as well.)

Pick and choose when to use this serve. Often you can tell by the receiver's stance and body language if he's looking to step around. Even if he's not stepping around, if a receiver looks a bit flat-footed, he's probably vulnerable to this serve.

Once you can do the serve effectively, it's time to make it even more effective. Here are two ways you can do that. First, you can use spin, either sidespin either way or extra topspin. If you do, make sure to put more power into the serve so that you don't sacrifice much speed for spin. Extra topspin pulls the ball down and allows you to serve even faster.

Second, contact the ball with the racket still aimed at the righty receiver's backhand, but hit it down the line with sort of a slapping sidespin (your racket going from right to left at contact). This is my favorite. Typically the first time I go down the line, I do it the "easy" way, changing the racket direction at the last second. Now they are watching for that last second racket change, so I don't give it to them. By keeping the racket aimed to their backhand, you catch them moving to cover that shot, and their forehand side is open.

Once you've done a fast down-the-line serve, the rest of the match the opponent is guarding against it, and guess what? All your other serves become a bit more effective. 

Have you ever studied your opponent to see exactly when in his strokes he commits to a specific placement? There really are two important points in the swing.

The first is when the opponent has committed to the placement, but you don't know where yet. Normally you would not react to this, but sometimes, against a predictable opponent (which means most players), you can anticipate. For example, if you serve a deep breaking serve to an opponent's backhand, most likely he'll return it to your backhand. So you might anticipate this, and at the instant when the opponent has committed his direction - but hasn't actually telegraphed the direction - you might anticipate his return by stepping over to attack with your forehand. Don't overdo this, but it's a definite tactical advantage if you can do this sometimes.

The second important point in the swing is when the opponent has telegraphed where his placement will be. You should learn to observe this so you can move the instant you can see the direction. Many players don't move until the opponent has hit the ball, but for the large majority of players, you can see where they are going by the time they start their forward swing. At the higher levels, many players learn to hide their direction longer and to even fake one way and go another, so against players like that learn when they have really committed.

The shoulders are often the giveaway for where a player is going on the forehand. Many players line their shoulders up early to hit crosscourt or down the line, and it's like they have a big sign across their shoulders saying where the shot is going. When you first learn the forehand, this is fine, but as you advance, learn a little subtlety and deception. For example, from the forehand side, rotate the shoulders way back as if you are going down the line, then at the last second whip about and go crosscourt. Or set up to go crosscourt, and at the last second rotate the shoulders back more so you can go down the line. Or simply cock your wrist back at the last second and go inside-out down the line. These are easier shown then explained - have a top player or coach show you how to do these shots.

There's a long history of players experimenting with how wide to keep their feet. There have been times where the trend was to keep the feet closer together, no more than shoulder width. However, that has pretty much died out. Suffice to say that if you watch videos of all the top players in the world, one thing that stands out is that just about all (I'd say all but I haven't had time to watch every single player) keep their feet rather wide. This gives them stability and balance when making shots, as well as lowering the center of gravity, which makes quick movements easier. There's a simple way to verify this, and see what the world-class players really do. Go to the ITTF world ranking list. Pick a player. Then go to youtube, paste the player's name in (and perhaps the words "table tennis" afterwards), and check the videos that come up. In general, the taller the player, the wider the stance, but even shorter players keep the feet wider than shoulder width. 

Many opponents push back deep any backspin serves, and will often even do so against a sidespin or even topspin serve by chopping down on the ball, often right off the bounce. And so you get a predictable backspin return off your serve.

If the opponent is going to give you these slow, predictable backspin returns, then you must take advantage of it. This means starting all of these rallies off with a serve and loop, either forehand or backhand. Simply decide you are going to do it, and do it. Yes, easier said than done, but the more you do it, the easier it becomes. You don't need a lot of speed as long as you get good spin, loop the ball deep on the table, and vary the placement to the wide backhand, wide forehand, and at the opponent's elbow.

Often it's best to serve short to the middle. By serving short, it makes it difficult for the receiver to attack the serve, and so you get those predictable long push returns. By serving to the middle, you cut off the extreme angles.

You should favor your stronger side - usually the forehand, though not always - but it's better to be able to attack from both wings when necessary than give away such a free attack. Don't over-anticipate the direction of the incoming push; wait and see the actual direction as a crafty opponent might aim one way and change directions at the last second.

How important is it to attack these long backspin receives? I've seen top players coach matches where they were literally confused that they had to actually tell a player to do this since it seemed so obvious and second-nature to them. 

If you have a tricky serve that opponents miss or pop up over and over, that's great. However, too much reliance on this can actually hold you back. The same tricky serve that your peers mess up against might be returned more easily by stronger players, including the ones you hope to learn to beat. For example, if you have a side-top serve that many opponents push back and so pop up, stronger players, especially after they see the serve a few times, might just drive or loop it, and suddenly your serve is a disadvantage. So instead of relying on winning off the tricky serve over and over, develop a good third-ball serve, one that players at nearly all levels will return somewhat passively, allowing you to attack. Then you can use the tricky serve as a highly effective variation that even stronger players might never adjust to.

Your typical third-ball serve is a short and low backspin or side-backspin, which is often pushed back, setting you up to loop (or in some cases, a regular drive). The irony is that if you do these serves well, opponents will tend to expect backspin, and so when you fake backspin but instead give side-top or no-spin (by contacting the ball near the base of the paddle but still exaggerating the serve motion), the opponent will often push and pop the ball up. Sometimes the no-spin becomes the main serve, with spin the variation, since no-spin serves are harder to push heavy or drop short than backspin, and if very low, are often harder to attack as well.