After years of trying every conceivable twine, string, and other modern tomato-securing convenience (what?) I finally stumbled on gift ribbon. For tomatoes in our bucket SIPs it's ideal, especially now when plants are tender and easily broken on gusty days.
Curling ribbon is waterproof, unlike twine, which absorbs moisture and holds it against vulnerable stems and leaves. It's also easy to tie into bows, allowing frequent adjustments as plants mature. Plus it's ubiquitous and tres inexpensive.