It varies from eccentric design to eccentric design, e.g., Co-Motion's pinch-bolt, Cannondale & Trek's wedges, Santana's dual set-screw, and the internal expanding models. However, the basics are the same....
1. Remove the cranks and bottom brackets
2. Mark the right side and then remove and disassemble the eccentric.
3. Clean & degrease the entire eccentric (disassembling as required) and the inside of the bottom bracket (BB) eccentric shell.
4. Inspect the eccentric and BB eccentric shell for any high spots or burrs that might prevent a tight seal between the two.
5. Apply a waterproof grease to the inside of the BB eccentric shell, the eccentric and it's constituent parts, and then reassemble and re-install (making sure the mark you put on the right hand side of the eccentric ends up on the right hand side of the bike).
6. Re-install the BB, using teflon tape if the cups don't have a thread-locking material and torque to the manufacturer's specs; drive side (fixing cup) first, then the non-drive side (adjusting) cup.
7. Re-install your cranks being sure to appy a waterproof grease to the crank bolts and torque to manufacturer's specs.
8. Re-install your timing chain, adjust the cranks to your preferred in-sync / out-of-sync configuration and then set the chain tension by rotating the eccentic as required to get at least 1/2" of slack.
9. Once everything is adjusted, tighten the bolts that secure the eccentric in place to the manufacturer's specs OR around 60 - 80 inch pounds (ball park). You can find a guide to bicycle torque values here:
http://www.parktool.com/repair_help/torque.shtml
10. If the eccentric still creaks, apply a little more torque to the bolts but be mindful that aluminum eccentric bolts can and do snap if you overtorque them.
11. If the eccentric still creaks make sure it's really the eccentric and not a pedal, your stem, or seatpost bolt... sounds travel in strange ways on a tandem.