Heat up the needle first to sterilise it, and then pop them. Drain the fluid, and cover with a blister plaster. Basically, if the blister pops by itself it is likelyto remove skin and not heal properly, whereas popping it yourself with a sterile needle and then covering it when youhave to (leaving itopen at night) means it will heal better.
You might also want to consider wearing better footwear, like properly fitting walking boots or trainers so that it doesn't happen againg. Plus you can sunbrn your feet wearing sandals, which isn't pleasant.