I get dizzy frequently too, its important to identify the triggers and also everything that makes it worse. Like heat, exertion, less frequent meals, low sugar, low BP, etc.
Since I am better informed now, I tackle it better.
Stop whatever you're doing and try to eliminate any of the factors that make you feel worse.
Sit or lie down as soon as possible, preferably in the shade or cool, sip fluids, carry a chocolate if possible for such situations, or a salty snack that you can access fast.
The idea is to be really quick, so you don't pass out. :-)
I started deep breathing to reduce anxiety/stress in general, and I noticed a Positive side-effect. :-)
When one is uneasy or unwell, the breathing naturally tends to become disturbed/faster. That sometimes magnifies the extent of the issue at hand.
So I've trained myself with practice - to close my eyes and Deep Breathe a few times, when I've exhausted all other options. Most of the time, it comforts me, and prevents any panic. That in itself, helps me handle the crisis better.
According to me, the deep breathing definitely helps, but I can only do maybe 5-10 deep breaths at a time, or it makes me dizzy :-) and defeats the original purpose itself. :-) Even few minutes of Yoga actually used to make me feel I would black out! :-)
(No offence to Yoga or those that practice it!)
As a general routine for health, 5 deep breaths in a row, every chance I get - works best for ME.
If it feels like a very tiring exercise to do - probably you should only try 1-2 deep breaths at a time. (That is how I started).
Andy this definitely has something to do with increased oxygen intake plus a lot more - that i'm not sure of. :-)