Hello mrsGembles
Oh daer you are one of the unlucky ones :( Im very sorry but ondanestron is the strongests of the drugs they give out, except steroids which is very rarely given except in cases of severe dehydration and malnutrition.
They could look at upping the dose maybe. Some people here have been on ondansetron the whole preg and can say what doses they have been on. Sometimes also people find a combination of drugs helpful, such as ondansetron with metoclopramide or promethazine or prochlorperazine.
I woudl say go back to the doctors simply in order to ask, and also to let them know that it's getting bad again. It's worth it because ... well, the last few weeks of a preg are heavy, no getting away from it, and you need all the support you can get.
At this stage im sure you've found out most of what works for you already food-wise :s But the advice given is to eat a little whenever you can, not big meals (Im sure you know that by now!). It's well worth looking at the links Lucinda gives in the OP, there's a raft of useful info on the top two sites in particular. some eating advice is given here on the preg sickness support site.
Are you actually vomitting a lot again? or is it mainly the nausea?
Oh, and the irritating old thing people tell you to 'rest, rest rest' im afraid is actually true. the nausea nad vomitting do get significantly worse if you overdo it (or if you're unlucky, do anything at all except lie in bed).
We're always here, inching towards the end of the misery ... and actually getting a baby at the end of it, which is somehow all too easy to forget when you're growing huge but are endlessly sick!
(and oh, the only use of ginger in severe cases of nausea is to relieve tension when too frustrated; insert up nose of well-meaning-but-ignorant person suggesting it)