I have a tomato plant that is now 6ish feet tall and still seems to be growing up and up. Is there a way to discourage this upward growth and keep it to a height that I won't need a ladder to harvest?
Unless perhaps it's a specialist variety then if it's 6ft then it has already put out more than four trusses - I presume. So you should have already pinched out the top to retard upward growth and force the plant to focus its energies on its fruit.
I work on the 4 trusses principle. However I didn't try to answer this question because I hoped for a more expert gardener than me to chip in and confirm it.
Simply pinch out (prune) the growing tips. This will also help the plant put more energy into ripening fruit.