In theory, cutting the flower heads off before they form seed will avoid the plant being weakened by making seed, but I've never bothered and haven't noticed the difference. Just keep the foliage growing well for the rest of the season and let it die down naturally in the autumn as this will put strength into the bulb for next year. Watch out for the lily beetle, a very small red beetle which makes holes in leaves and then lays eggs, as this will weaken the plant. The damage mainly comes from the little grey grubs which devour most of the lily foliage. Manual removal is the best strategy, though messy.