Here's what I did, and it seemed to work pretty well...

I've got a Dewalt 618PK, which has a 1/4" pilot bit and a 'cone thingy'
on it. the idea is to put the plate on, then the 'cone thingy',
pointed toward the router, and push the cone down the pilot bit until
it centers the plate or base. So, I:

1. Removed the base from the router and placed double-stick tape on the
bottom of the router.

2. Chuck the pilot bit.

3. Lay the plate down gently on the router (no pressure).

4. Put the cone on the pilot bit and slide it down until the plate is
centered.

5. Remove the cone.

6. Put doublestick tape on the router base, and lower it gently onto
the plate.

7. Put the cone back on and slide down the pilot bit until base is
centered, then press down on the base to secure it to the plate.

8. Drill the holes.

I believe the 'cone thingy' may be available seperately from Dewalt,
but I'm not positive. I saw something similar at

http://www.davehylands.com/Wood-Work...Centering-Jig/

and another option at

http://www.mlcswoodworking.com/shops.../routacc1.html