Use the "Both" position for take-off and landing. In the cruise select the higher tank until the fuel is "balanced" then back to "both". On a long flight you might have to "balance" the fuel more than once. I use an egg timer to remind me to put it back to both.
However, If the lack of fuel is becoming an issue with both tanks below 1/4, it might be a good idea to select left or right tank. Run the engine on that tank until it starts to cough (run dry), immediately select the other tank and find somewhere to land!!!!
Bob.