Ok this is old school for cabinet makers but it works for this application quite well!
Take a look at this illustration, it's from pocket door hardware for cabinets...
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quite simply what this is is a moving knot, two cables pull against each other to effectively prevent one cable from moving without the other cable moving. I was unhappy with the play I had on one side of my machine caused by my placing the Y screw to the side of the machine, I did this because I wanted open clearance in the bottom of the machine in case I needed to place a large object under the gantry

It cost me about $40 in parts to get my machine rock solid I think it was worth it and this would be a good fix for those of you whose machines work great but are just a bit wiggly saves having to rebuild/rework much.

I picked these up at lowes they are patio door rollers they make awsome pulleys as they are grooved nicely, have an internal bearing and built in standoff.
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after cutting them out of their brackets (i'm kinda interested in these as a v-bearing substitute as they fit a 1/4" steel rod nicely and were designed to hold up a 100lb door!
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See what I mean about v-bearings huh? huh? and if you use the mounting bracket they come in it has a tightening screw that would have been used to raise the door up to tighten it into the track! You could use it to take out the play in your axis'
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Anyways, Back to what I was describing. I used (4) of these rollers,(15') of 3/32" steel cable (plastic coated),(4) 1/8" cable clamps and (4) 5/16" X 3" eye bolts for tightening up the cable.
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I stacked up 2 roller wheels 1 stack on each side of my gantry rails.
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The rollers have a slight hub in the center that acts as a standoff allowing them to turn seperately from each other.

By passing the cable around the pulleys and attatching them to the base as illustrated in the diagram at the beginning of my post you end up literaly tying your gantry to the base! I just set mine up tonite and it is now solid!

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Last pic!
forgot to show the roller up close
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