support

[dice]

Itoero, I did not notice that you are growing single-stem
tomatoes in a greenhouse. I think hanging strings from
some overhead support and either wrapping the string
around the stem as it grows or using tomato clips to
clip the stem to the string is most popular for growing
single-stem indeterminates in greenhouses.

Here is a picture of the type of clips growers commonly use:
http://www.hydro-gardens.com/growsup1.htm#plantclips

(But of course you can make some equivalent from anything. dcarch
once posted an idea to make plant ties out of plastic shopping bags.
He would fold the bags over the long way several times, then cut
them into 1-inch wide pieces with scissors. Unrolled, each 1-inch
piece would make a plant tie that was wide enough to not bite into
stems and elastic enough to stretch in a wind without snapping off
stems. And free, of course, for anyone that brings groceries home
from a grocery store in plastic shopping bags.)