We used baler twine from D rings to bit when I worked at riding school but I prefer the idea of this sort of daisy rein that goes from saddle to bridle/pole http://www.mhts.co.uk/daisy-rein-152-p.asp
Slightly more sophisticated is to use a thin nylon rope with clips at both ends - but as with grass reins from bit via the poll to d rings on the saddle - don't use with a young pony. I have seen people use quite tight side reins - but only good on bonb proof lead rein pony.
we have daisy rein that came with prev pony, ive never seen the need to use (if she dumps me i need to sit tighter next time) but they clip to either side D ring & then centre of bridle and either side by the cheek/throatlatch split.