Take drinking straws and cut in half lengthwise.  Attach the halves to a piece of foam core with short sequin pins.  Now you can lay out your beads (the large ones for the necklace) to determine placement and they will not roll all around.

To prevent your seed beads from rolling all over, put them on a piece of felt.

Mary from somewhere in cyberspace has this to add:
Try double sided plastic carpet tape (scotch tape is too sticky) placed on a piece of 1" thick styrofoam (thinner would be too weak).  Open a box of beads, string all of them and then just put a row of beads on the tape and work from there.  You could put some of all the beads needed for a project on this tape.  Also, try using a #12 quilting needle for both your petite and regular seed beads.  This needle gives you more control with a shorter length which may help you not to stick yourself as much as with the long beading needles.

Use one side of corrugated cardboard and lay out your beads in each of the trenches to design a beaded pouch or similar type project.