So sorry Pollydon, cystitis is just miserable isn't it? I suffered for ages but now have a routine and products that have kept me cystitis free for about a year so far (used to get attacks with same frequency as you.)
My list is similar to some of the PPs but FWIW this is what's working for me:
-Waterfall D Mannose- I take three tablets an hour before sex (if we are organised!!) and always three immediately afterwards. I take three immediately whenever I get that telltale twinge and it seems to knock infections on the head. I've got rid of established infections with it in the past too, and it is nice not to have to rely on antibiotics.
-CranFlora tablets with Probiotics (Acidophilus is the best probiotic for this according to my Health Store lady!)
-Good Vitamin C tablet for immune system
-Wee before and after sex, also wash before if you can, and after with fragrance free soap.
-Showers never baths, fragrance free soap (sanex zero is good), no chemically bubble bath/shower gel/etc.
-Drink lots of water, especially after sex.
-Cut out caffeine. I thought I would HATE this but I've got used to de-caff tea.
-Avoiding too much acidic food/drink at once- so I still drink wine, for example, but I never have wine plus citrus fruit plus an acidic dressing on salad, etc.
-This one is hard- cutting down on sugar!
My biggest tip is D-Mannose, it really does work, but it is pricey.
Good luck, it is so horrible to live with!
PS I'm sure you know but just in case: do not drink cranberry juice when you actually have cystitis- it will make your urine more acidic and the bacteria will have a party! I'm not convinced that cranberry juice works at all as prevention- too much sugar and not enough cranberry- try the CranFlora or similar tablets.