I have a 4x2 Nissan Xterra 2002 V6 that I bought at 20,000 miles in '03.

After few months of purchase, I noticed the rough ride on regular roads and bought the Bilstein shocks (B46-1099-H1 and BE3-6422-H0) for front and rear. This had improved the ride quality a bit.

This is better than OEM but still its quite bumpy on roads.

Any ideas how I can smoothen the ride.

Inputs appreciated.

SR.

Remember that it's a truck and complete "smoothness" can't really be attained. I too have Bilsteins on my '04, which totally smoothed things out, compared to the stock shocks.
You may want to check the bumpstop clearance in front. If they are touching the upper control arms, everytime you hit a bump, you're gonna get jarred around.
Hope this helps.