Step up DC/DC 5V -> 6V
boardbite's TPS61040 is just one of many step-up converters which will do the job. But it costs 1.88 dollar in 1s at Digikey, while the cheapest I found is only 70 cent. That's the Semtech SC4503. 



This is the typical application schematic from the datasheet. Again, this will be similar for many other step-up switchers. The SC4503's reference voltage is typically 1.25 V, so for 6 V out you have to set R1 to 190 k&Omega;. At 12 V out and 200 mA the SC4503 will have a 90 % efficiency, for 6 V you can expect a slightly higher value. It can supply more than 1 A of output current, so you'll have lots of headroom. Comes in a SOT23-5 package, and the 1.3 MHz switching frequency means you only need a small inductor.