Hi
Guard traces are usually not so effective for high speed signals.
Best is to separate the pairs or decrease the height above the ground plane
From "High-Speed Digital Design, A Handbook of Black Magic" page 201
"As a rule of thumb, the coupling between microstrips is halved by inserting a third line, grounded at both ends, between them. Their coupling is halved yet again if the third line connects through vias to the local ground plane at frequent intervals."
Between two lines:
Crosstalk < 1/(1+(D/H)*(D/H))
D = distance between the lines
H = Height above the ground plane
You can find more information of this at:
**broken link removed**
**broken link removed**
or in the book mention above by Howard Johnson