In my opinion, it would be easier to buy some lumber the same size as your floor joists and cut full blocks to nail between your joists. If you insist upon using the x members, I don't know the recommended spacing.....I would probably put them every 6' - 8' or so.
What you are talking about, as sebd it all stated, is "bridging"....or more specificcally: "Diagonal cross bridging".
Generally, it is installed at ''mid-span'' of the joist member's length. Tho, adding it at more frequent intervals is not unusual.
Here's some more information about this:
"Some carpenters and builders may also use diagonal bridging in between floor joists. The individual pieces of bridging that create the letter X in between the joists must not touch one another. There should be a spacing of approximately one-half inch so that they can move independently as loads are placed upon the floor joists. If the bracing touches and moves, it can also produce the elusive squeak that causes the grimace on your face." http://www.askthebuilder.com/NH048_-...y_Floors.shtml