If you get a blister, should you pop it? This is a question of finding the least bad option, as there is no ideal solution, other than preventing the blister in the first place (Blister Prevention).
Popping a blister is a bad idea because it introduces the possibility of infection. A popped blister may well refill, which is the worst of both worlds. Popping a blister may not reduce the discomfort in the way you expect, and can make it worse. The fluid can act as a cushion to protect the raw flesh under the blister. Once the blister is popped, the two surfaces will then rub. I have also found that leaving the blister intact improves healing time.
The only reason I would pop a blister is when pressure will cause the blister to spread. If you have to pop a blister, you should sterilize the area first. Puncture the blister at the edge, ideally at the edge that will be lowest most of the time so that gravity aids drainage. I use hypodermic needles for bursting blisters as they are very sharp. Multiple punctures may be needed to provide sufficient drainage. Use a sterile dressing to push out the fluid, taking time to ensure you remove as much as possible. Cover with a sterile dressing; I have found Band-aid blister cushions work well for me. You may need an adhesive promoter such as tincture of benzoin to help keep things in place. Be careful with tincture of benzoin. If you put it near the punctures in the blister it can seep inside and be quite painful. Also, putting tincture of benzoin on the blister itself will mean that the blister and the dressing are bonded, making the dressing harder to remove. If possible apply tincture of benzoin only to the surrounding skin.
There are two techniques I will mention, but would not recommend and have never tried. One is to thread a strand of cotton through a blister so that it continues to drain. I feel that this increases the risk of infection significantly. The other technique is to inject tincture of benzoin into the blister after it is emptied, which glues the detached skin to the underlying flesh. I hear that this is amazingly painful and I am not sure of the risks in doing this.
The alternative to popping a blister due to pressure is to build up the area around the blister so that the pressure is relieved. Moleskin can be cut out in the shape of the blister, forming a doughnut shape that reduces pressure on the blister if it is small enough.
The best place for blister care supplies I found is Zombie Runner. I would also recommend the book 'fixing your feet' by John Vonhof.