Sasben has the right idea.
Pre-flight you must ensure that the fuel from both tanks is available. otherwise the first inflight tank change could get exciting.
Buy switching before the runups you are ensuring that the fuel in the line is burnt and that you are now drawing from the other tank.
As a side note, if you want to know how long the fuel in the line will last try turning the selector off on the ground. At idle you will find it can run up to a minute!