San Rensho 

Try this. Use the middle chainring and try to keep your cadence the same, or within a range, and vary the cogs in the back according to speed.

So at a dead stop, you are in your smallest gear. As you start to move, concentrate and increasing your rpm's. When you reach your target rpms, shift up a gear and then continue to increase rpms and shift until you feel yourself doing too much work.

As you slow down, downshift, and right before you come to a complete stop, get back down into your smallest gear.

Its very similar to going through the gears in a car. You never start off in 4th gear and you don't want to be going 55 mph in 1st gear. You Start in first, go up through the gears as you speed up and downshift as you slow down.