The first colonic irrigation I did was not very pleasant. I learnt later that with a little planning any discomfort can be avoided. So directly before, and for 2/3 days after you should avoid the following:
Alcohol, caffeinated tea and coffee, carbonated drinks, dairy products, desserts, fried and fatty foods, flour products, ice cream, processed junk food, sugar, wheat products
gas producing foods:
Beans, broccoli, brussel sprouts, cabbage, cauliflower, onions, peppers, soya products, raw apples
Also make sure you drink pleanty of water before and after (not just before). You should stop eating a few hours before the colonic. I find it easier to fast that morning until it is done, but oats are a good breakfast to have beforehand if you are hungry, and a salad for lunch if you are having it done in the late afternoon.
It is a good idea to go for the following before and after:
vegetable juice, chamomile and pepermint teas, fresh vegetable soup, fish and poultry, green leafy vegetables, seasonal organic vegetables, seeds (pumpkin and sunflower)
This good advice was given to me at the clinic I attend: http://aquadiaqua.co.uk/ where I did not experience any pain, having avoided the gas producing foods outlined above.