After lilies have finished flowering, it is important to take care of them properly to ensure they bloom again next year. Here are some steps to follow:

1. Cut the flower heads off your lilies and allow the foliage to die back naturally. Do not cut the stem back until it becomes hollow and brown, as this will feed the bulb for next years flowering.

2. Leave the bulbs in the ground or in your pots as they like a cold dormant phase throughout the winter (they can tolerate -20°C). Ensure they have plenty of drainage so they dont get waterlogged as this will cause the bulbs to rot. You can tilt the pots on their side or place them under shelter to prevent this.

3. Deadhead the spent flowers by picking them off by hand or cutting them off using a pair of shears. This will prevent the flowers from producing seeds and diverting energy away from producing more flowers.

4. Prune the stems back to the ground using shears once they become hollow and brown. Be careful not to prune lilies prematurely as this can diminish next years growth and flowers.

5. Disinfect the tools you use each time by wiping them off with a mixture of 1 part bleach to 9 parts water or rubbing alcohol. Wipe down the blades between each cut to prevent the spread of disease.

By following these steps, you can ensure that your lilies will bloom again next year and continue to thrive.