Put very simply, warfarin acts to inhibit the Vitamin K - dependent clotting factors (Factors II, VII,IX and X mainly) produced within the body. The more warfarin, the greater the inhibition, the higher the INR (International Normalised Ratio).
So, any food or food group high in Vitamin K has the potential to interfere with the action of warfarin, thus lowering the INR.
Such foods might include ( this is not meant to be an exhaustive list, by the way) Spinach, Brussel Sprouts, Kale, Asparagus. Beef and Pork liver are high in Vitamin K also. Cranberry and I think Grapefruit juice have also been implicated. This does not mean, by the way, that if you are on warfarin you have to avoid these foods entirely - just don't overdo them :)
Vitamin K is synthesised within the body in the gut by naturally occuring bacteria - So a course of some antibiotics has the potential to interact with your warfarin therapy, further reducing the Vitamin K within the body, and therefore potentially elevating the INR.
You also need to be aware of various commonly prescribed drugs and their interactions with warfarin - such as ( for women of child bearing age) oral contraceptives,carbamazepine, ( reduce the effects of warfarin), and aspirin, paracetamol, glucosamine ( all likely to increase the effects of warfarin)
A special class of its own is reserved for alcohol. The most common reason for unexpectedly high INRs is overconsumption of alcohol. The second most common is overconsumption of aspirin or paracetamol based painkillers.
I would strongly advise you to talk with either your GP or the practice nurse who I am sure would be only to happy to advise you much more comprehensively that we can do here.