Beads, Beads, Beads.
Tips on choosing elastic cord
Once you have chosen the beads that you would like to use, for most people this would be a round bead 6-8mm in size, one could go smaller or larger, the issues that you may encounter would be, on a smaller bead such as a 3-4mm the size of the hole could become an issue and you may not be able to get the elastic through. On a larger bead 10-12mm in size the elastic may not support the weight of a bead that size and become loose and saggy.
Choosing the elastic, my personal preference is the nylon beading elastic by Beadalon called Elonga, it is a flat elastic that looks like dental floss. We carry it in 3 different thicknesses 0.3mm, 0.7mm and the most recent one is a 1mm thickness. This product is not colored, it only comes in what would be best described as clear/white. I always suggest that you use a needle when using this product as it will make your stringing much simpler.
Beadalon also makes another line of elastic called Elasticity, this product is clear and round in shape, like a strand of spaghetti, and about as limp too, I might add. It also comes in 3 different thicknesses, 0.5mm, 0.8mm and a 1mm. I am not a fan of the Elasticity simply because the surface is very smooth which can make tying your knot a challenge, one usually will need to secure the knot with a drop of glue or clear nail polish. Then there is the issue of the knot being to large to fit into the hole of the bead in order to hide it.
Regardless of which elastic or the size of the bead that you choose to string, I always try to suggest to customers that they should go with what makes them feel comfortable as it should be a fun and enjoyable project.