Had a useful tip from our CFI for changing tanks (once in the cruise and taking into account crossing water, built up areas etc)
Use the minute hand of your watch - if it points to the left (i.e. between 31 minutes and 60 minutes - use the left tank and if it points to the right (1 minute to 30 minutes) use the right tank.
Means you will regularly cycle between left and right every half hour or so.
Personally I always turn on the fuel pump before changing tanks and then off again whilst checking the fuel pressure.
As has been said above NEVER change tanks after power checks until you are confident you can glide to an acceptable landing site. I usually come "off pump" when leaving the circuit, and "pump on" when rejoining.