Oral contraceptives have a high level of progestogens, which relaxes smooth muscle which is muscle that you do not consciously control eg bowel. It slows down the speed at which your food travels through the intestine. When it takes longer to get through your colon, more water is reabsorbed making stools harder. This in turn slows down transit further.
Soluble fibre is recommended (ie oats, beans, fruit and veg, stewed fruit eg apples, prunes, apricots - incidentally these are supposed to help with cholesterol too)
Plenty of fluid
feet on a stool, or equivalent
exercise

encouraging an urge at the same time daily

coffee, chewing food well to promote peristalsis, regular mealtimes.
Pre/probiotics etc
Pre-empt constipation by taking laxatives when you know it will happen.
Movicol works well

(I was recommended senna and ortis cubes but they both give me tummy pains.)