1. Unisexuality. The flowers are unisexual i.e. stamens and carpels occur in different flowers. They may be present on same plant e.g., in maize or on different plants e.g. in Papaya. The first condition called monoecious and second is called dioecious. 

2. Dichogamy. When stamens and carpels of bisexual flowers mature at different times, it is called as dichogamy. If stamens mature first it is called protandrous condition, or if carpels mature first it is called protogynous condition. Dichogamy prevents self-pollination and ensure cross-pollination.

3. Self sterility. In orchids, pollen grains of one flower do not geminate on stigma of the same flower. This is called self-sterility. The pollen grains germinates on the stigma of some other plant and thus ensure cross-pollination.

4. In some species the anther and stigma are placed at different levels so that the pollen does not come in contact with the stigma of the same flower.

5. Plants bears flowers which have characteristics to favour cross pollination  like attractive color, smell, may produce nectar. be very light and feathery pollen grains, sticky stigma etc .