1) Fix leaky taps, or place buckets or old ice cream containers under the taps to collect water that would normally go down the drain. You can then tip the water collected onto the garden.
2) Have shorter showers. An easy way to make sure you're not staying in too long is sticking a five minute egg timer to the wall.
3) Water from your water bottle that you don't drink through the day can be poured onto the garden rather than tipped down the sink.