I work in 3 magazines as a retoucher, and every day I have tons of photos and pictures to be proсessed, retouched, isolated and so on. When I started working, I didn't know how to isolate objects correctly and accurately. I had to read a lot of articles - some of them were useful, others not. But even if you read all the articles in the world about isolation and don't try even once to do it yourself, you won't know what it is.
I differentiate between several types of pictures to be isolated.
1. Objects / people shot on a white or monochromatic background.
In this case you can use a Magic Wand Tool to isolate such objects. Be sure not to select some areas you don't want to be selected. Use Lasso Tool to add/subtract to/from selection.
2. When you have to isolate an object from a natural background, you can use several ways to do it. But I mostly prefer to use a Pen Tool as it provides me with the most precise contour of the object, moreover, I can correct every curve in any moment I need.
In this picture I wanted to preserve sugar crumbles, and I had to trace the biscuits with a Pen Tool, then make a selection from the path, and delete the background, saving the Clipping Path for designers who'd use it.
3. Sometimes I shoot objects on a white background, which doesn't look white before post-processing.
In this case I create a new Adjustment Layer for Levels, choose a white dropper and click on the grayish background. It becomes absolutely white. Then I erase the areas I don't want to be so much high lightened using a mask of the Adj Layer.