I was worried when we first got our kefir that we would somehow stuff it up and ruin it. Fortunately so far the kefir has done really well and the grains have grown in size and number compared to the tiny bit that we started with. What do we do with all this extra kefir? If you leave all the grains in the milk each time you change it, the milk kefir will start to be ready too quickly (usually we leave is a few days until we see the milk has thickened, but if there's too many grains it will only take one day and we have too much kefired milk to use up!).
First I recommend that you start a back-up kefir. You can put some spare grains in a jar with some milk in the fridge and leave it there for weeks, just in case your main kefir has a problem, then you can easily start again. You just have to change the milk every 6 weeks or so (and use the kefired milk as normal). We now have two back up kefirs, just in case!
I also recommend giving kefir away to anyone who will listen! The more friends you have with an active kefir, the more likely you can borrow a few grains if you ever need some :)
The last solution if the kefir is thickening too fast, is to put the main kefir in the fridge too. This happened to us on some really hot summer days. We just got it back out of the fridge when we were nearly out of kefired milk, and it was ready again by the next day.
Jars of kefired milk, straining the kefir grains out of the milk, the back up kefirs, some milk (thanks Bella!) and some cream skimmed from the milk