If you are using Photoshop and are selecting a hard edged object (like a car) then you may want to try using Photoshop's Path tools.
They will let you define a clean curved edge that can be hard to replicate by using lasso tools or other hand-painted techniques.
Eg.
In Photoshop I would copy the layer your photo is on, desaturate / convert to b+w that whole later, then apply a layer mask to 'cut out' areas and let the original colour layer show through.
This way you can easily combine techniques like Paths and hand-painting to produce your layer mask. Its a less 'destructive' way of working that means you can easily undo changes you have made
