Well, there are no shortcuts, but some key issues are:

- knowing your scales and arpeggios (major and minor)
- knowing the chromatic scale up and down the entire range of the instrument
- developing your ideo-kinesthetics (your ability to "feel" where you are on the keyboard, so you don't have to keep looking down at the bars...and up at the music...then back down to the bars...then up to the music...then the bars...back to the music...etc., etc., etc.)
- slow practice.  Lots and lots of slow practice.
- if you find yourself hitting the *same* wrong note in a given phrase, learn the phrase with the wrong note - it will train you to recognize by muscle memory what it feels like to hit that wrong note, and that will help you to correct yourself before the mallet actually hits the bar.  I know this one sounds backwards, but trust me, it works.
- whatever you learn, learn it in all twelve keys.

I'm also a believer that one should start on a mallet instrument using only two mallets, to help one learn his/her way around the keyboard shapes, before addressing four-mallet technique.  If you haven't done so already, don't be afraid to spend some time learning two-mallet pieces and exercises.  It's not a step backward from four mallet playing, just a different technique for playing the instrument.

Just as James said ... S L O W methodical practice.

Also, developing your feel for the keyboard layout by NOT LOOKING AT THE BARS! What I did to train myself was the following:

First, I lowered my music stand so that while looking at the music, I could see the entire instrument via peripheral vision. Even if you can't see the lower manual, you can see the upper manual (flat/sharp) bars; from there, they can be your guide.

Secondly, I would close my eyes when I practiced. Obviously this means you no longer need to look at written music. If you can't play with eyes closed, try covering the keyboard with a sheet. For me, this allowed me to get the ideo-kinesthetics thing happening, as James mentioned.

Be sure that you spend a good amount of time making sure you are striking the bars in the best possible place. It's not enough to just hit the right note, but you need to make sure that you are getting the best sound from the instrument. I always go for the center of the bar. If this isn't possible due to speed or awkwardness with the four-mallets, strike the bar as far away from the node (where the string runs through) as possible. This means if you play on the edge, play on the VERY EDGE. There's no sense training yourself to just hit anywhere.

Lastly, be sure that you work on your body position, posture and play the full length of the keyboard. If your body doesn't have a "home base" on the keyboard, you are not going to have consistancy in your ideo-kinesthetic development. I keep my body attitude the same the entire time I'm playing (as much as possible). When I have to move my body, I try and maintain the same posture and attitude towards the instrument ... and just glide along ... something I call the "marimba shuffle". The same applies to the vibraphone, except you must keep your one foot on the sustain pedal/bar.

In regards to playing the full length of the keyboard, I mean just that. I practiced chromatic as well as all major/minor/etc scales the full length of the keyboard ... going as low and as high as I could go, regardless of what the starting not was. An example: if playing a C Major Scale on a standard vibraphone, I would start on the low F, which is in the key of C and is the lowest note on the instrument. I would practice the C Major scale up and down instrument. In this case, it sounded a bit like I was playing an F Lydian scale, but that's okay. The point is that you play EVERY NOTE possible within the scale/chord/mode/etc that you are working on. This really helped me in my accuracy. I also applied different stickings to this; starting with the left hand, then the right hand, then left hand on the upper manual bars with the right hand on lower manual bars, etc, etc.

I would also recommend the Phil Kraus books, Modern Mallet Method, Volumes 1, 2 and 3, as an excellent source for new and fresh ways to play your scales/modes/chords/etc. The varied patterns and systems that Phil uses helps make the practice more musical, challenging and applicable. Volume 3 deals with four-mallets ... which is what your original question was about right?  

Felix, I hold four or more apparatus (sticks, mallets, etc.) when I play the drumset sometimes. The application would be when you want to play something simultaneously, and two hands is just not enough. I also do it when switching sticks is not an option, like needing to do a suspended cymbal roll and sticks are too harsh. There's hundreds of applications in a multi-percussionist set-up ... but for drumset, being that the dynamic level and intensity is usually very high, the four-mallet or multi-apparatus techniques just won't work. Now I do hold multiple sticks, multi-rods, brushes, etc., as a bundle to create aunique sound.Kind of like what Gadd does on some of Paul Simon's songs. I can discuss it further, but I think you get the point.

Felix - I use four malelts in my hands (possibly even more) when playing broadway shows. Sometimes, there's just no possible way to switch, even with double headed sticks.

By the way Felix, sometimes I'll play drumset using a 4 mallet technique using brushes and sticks with the brushes on the inside and sticks on the outside. It's interesting...

Now ... in that same era, I saw Lionel Hampton play with three (or four) sticks on his kit. He always seemed to use three mallets on the vibes, but I can't remember for certain about the drumkit.

You'd think holding that many sticks, and standing on his floor tom would just be a little too dangerous ... but I guess not.  

I used to work with this guy when I was at Disney - he developed and marketed his 4-stick drumset concept, calling it Quadragrip:

http://quadragrip.com/pages/q4t.html

Yes sir ... saw Hamp several times live and he would pick up an additional mallet from time to time ... so he held three. He always looked awkward ... and only held the additional mallet in the left hand, which makes sense.  I've also seen him do this when I've seen him on TV.  I don't remember ever seeing any footage of him do this early in his career ... but just in the last two decades.

I would be interested if any other people can confirm what I'm saying. Check around James and let me know what you find.

James is right, however, Lionel Hampton was principally a two-mallet player.

Leigh Howard Stevens book, Method of Movement.  Buy it, read it, over, and over, and over, and.....

Everybody has made good points.  I would like to reiterate playing all exercises in all 12 keys, but even take some exercises a bit further by going into nataraul minor, harmonic minor, melodic minor, or even the modes(especially useful for jazz).  

Another REALLY important part of playing keyboard percussion is the type of technique.  Just like drumset, keyboard perc can be played with multiple grips, stroke techniques, etc.  For marimba, i like the Stevens grip with nice firm birch handle mallets(non-lacquered due to my sweaty hands!).  For vibes, i prefer the Burton grip, and like rataan handles, but not too flexy.  I hate fiberglass handle mallets altogether.

As Bart said, it is very important to work on where to strike the bar.  Although I personally disagree with Bart on playing in the center of the bar.  You should realize what parts of the bar get the fullest sound, the deadest sound, etc.  I.E., right above the string is a nodal point, which is why they put the string there, it is the most dead part of the bar.  The very center acts as an antinodal point, this can also be quite dead.  I like to play the lower register(white notes) just off center, and the upper register(black notes) on the very end.  I beleive this is where the most full sound is acheived.  

Most importantly, have fun on this instrument.  Keyboard percussion can be LOTS of fun and can definitley expose you to more musicality, something we ALL need!!

Bart,
I know what you meant, and I wasn't trying to imply that i am all-knowing(although, ahem, i am).  I just used this forum topic as an a place to voice my opinion because i do see a lot of players hit dead center and not pay attention the sound.  That's all.  

As for the anti-node, i was told this is what is called.  That may be wrong.  The prof explained it to me as being where the soundwaves cross and the the nod being where they are furthest apart.  I have not done research on acoustics, so this may be something that is wrong that I accepted as being right.  That's all!