First, let's address the beading wire. The wire you choose should accommodate the weight and the hole size of the beads. Choose the largest diameter wire possible, on both accounts. Accu-Flex® beading wire offers tensile strength, (meaning it has been stress tested to ensure its strength), a soft drape and it comes in good variety of colors.
For best results, work with a suitable size crimp bead (one that is large enough to accommodate the wire passing through it two times) and a crimping tool. Accu-Guard™ wire protectors or French wire protect the beading wire as it passes through the clasp. Crimp covers complete the professional design by disguising the crimp bead as a smooth round bead.
When finishing off a beaded wire design with a clasp, give some slack to the length of the wire. In most cases, the beads will need a little bit of room distributed among them, so they can move and work with the soft drape of the wire. To achieve this, don't pull the ends too tightly as too much stress on the wire can cause it to break. Consider a color or metal finish Accu-Flex so it blends with the beads, and then the little bit of wire showing will complement the design.
Second, with regard to stringing with thread. There are a good number of styles of thread to choose from, all of which fall into two broad categories: natural (silk) or synthetic (nylon and polyester). The synthetic threads are strong and will fray less than silk. Silk offers a very soft drape, particularly when hand knotted or with the help of The Bead Knotter™. Silkon® offers a similar drape yet has the strength of nylon fibers.
Techniques for adding a clasp include knotting and gluing for silk, knotting and burning the ends for synthetics. Whichever thread you use, you may wish to knot the ends and then hide them inside bead tips or Hot Tips® designer bead tips. Bead tips have a metal hook on the end which is then used to connect the ends of your design to the clasp. You could also cover the thread that passes through the clasp with French wire, similar to the function of Accu-Guard wire protectors (used when working with Accu-Flex).