My tennis shoes give me blisters

Why do all shoes give me blisters, even when I take every precaution to prevent it?
I'm getting pretty frustrated with trying to buy shoes ever. My problem is that without fail, any pair of new shoes will give me blisters. The way I have dealt with this is by simply suffering through the blisters until I get thick callouses on whatever part of my feet that particular shoe wants to blister, but every time I get new shoes (which is rare) they blister some other, new part. I get blisters on the bottom of my feet, on the sides of my feet, on my toes, on all parts of my heel... you name it, it'll blister.

I'm getting very tired of this, and I don't know what to do about it. I have to suffer for literally months before I can wear a pair of shoes without considerable pain and some bleeding. I can wear a pair of shoes for one day and then I have to switch over to a pair I've already broken in so my wounds don't get irritated and infected.

I'm finally asking this question because I wanted to start walking daily but my old sneakers are completely unusable. I got new ones, but I can't walk every day because of the damn blisters.

- It does not matter if I buy men's shoes (I am female) or women's shoes.

- I do not wear heeled shoes, or strappy shoes, or any other torture contraptions they sell in the women's department that I would expect to give me blisters. I wear loafers or sneakers, and occassionally a pair of Tevas. (The Tevas gave me a blister on the back of my heel, even. Seriously.)

- It does not matter if I spend an hour trying on all sizes, with the help of sales associates, to make sure I get a proper fit. This past week I went into a Puma store to get running shoes, and the associate was very patient, knowledgeable, and helpful. The shoes seemed to fit alright in the store, but when I went to actually use them, I have horrible blisters. I can hardly walk in them, much less run. I already returned two pairs of shoes because of this, now I've given up.

- I don't have particularly weird feet or anything, or at least as far as I can tell. I'm a size 8 in women's (U.S. sizes). When I have gotten blisters I have tried getting both larger and smaller shoes of the same styles - I just end up with blisters in slightly different places.

I have tried the following:
- Putting band-aids or cloth over where the shoe wants to blister me. This makes no difference, because it just rubs the bandaid/cloth against it.
- Wearing different kinds of socks, different thickness, different material, etc. Doesn't matter: blisters.

I have literally never found a pair of shoes that does not give me blisters. Flip-flops give me blisters. Sandals give me blisters. It's ridiculous.