How does something so simple as asking: Wouldn't it be nice if...? help you release resistance?

It makes you change your mental focus.

Can I do it for someone else, say:

"Wouldn't it be nice if my mother were feeling better today?" Would it work for her?

Her thoughts are her own, but it will change your own mental attitude and energy, and that will have a positive influence on her.

Should I do it for one topic at a time or I can mix up topics making a looong list of things that are actually bothering me/making me uneasy?

Techniques like the Focus Blocks method and the Wouldn't it be Nice method seem to work best on one topic at a time, largely because human beings are really capable of focusing on only one topic at a time. But you can make your long list, and work on each topic individually.

Mentally or written down?

Writing things down is a powerful technique. It goes from the brain to hand to paper to eye, and back to the brain, reinforcing the mental concept being worked on.

For how long would it more fit to keep on doing it?

Until you feel better or feel a sense of relief/release, or when you no longer seem to get forward progress, at which point I would encourage you to work on something else for awhile.

Stingray and Vesuvius have answered your questions nicely.

I just wanted to add something of my own.

The "Wouldn't It be nice if...?" process is excellent. If I am out and feel myself beginning to dip I catch myself and use it to maintain my state of feeling good...or even to pick myself up when I am feeling a bit down but am out and don't have lists for focus blocks available. It is my main "while living life" process.

That said, I rarely use it on any predefined unwanted subject of mine. I use it purely to feel good. And when you have the statement 'wouldn't it be nice if...?' this is easy to achieve. You can imagine the most 'ludicrous' things (according to your current beliefs) with it, that you wouldn't really expect.

For example, just earlier today I was getting a slight dip in my mood and I just started using this process to get my mood up.

For example I thought "wouldn't it be nice if when I got to my place, was cooking sausages and offered me some for lunch?"

"Wouldn't it be nice if I heard how such-and-such project went?"

"Wouldn't it be nice if was lying naked on my bed waiting for when I got home?"

(ok, those aren't particularly ludicrous examples, but I don't want to appear too crazy ;))

And so on. Those thoughts got me feeling really good...they were 'random' and their only intention was to have me feeling good. That's really important. Stingray says it so much, but I've only recently realized myself how important it is.

The first two of those examples actually manifested as soon as I got back to my place. Alas, the last one didn't. But no matter. :P

For those interested in this process I want to suggest a variation that I think is much more effective in some vibrational states.

As Stingray explained in his answer, this question bypasses resistance or "tricks you into releasing resistance". If you combine this "tricking effect" with the power of appreciation, you have a more powerful process. Usually true appreciation works only when fully in the vortex. But with this a slight change, appreciation is often possible in lower vibrational states.

I've tested this successfully many times and it seems to work best for me when I'm feeling at least neutral on the emotional scale (and I think it works smoother when feeling hopeful).

So the question goes like this:

"Wouldn't it be nice if I could wholeheartedly appreciate... such and such?"

What a vast collaboration of helpful and honesty answers! This Inward Quest thing really rocks!

Here's my variation on it that I managed to apply today with feel good results.

I get out a word document and randomly begin the "wouldn't it be nice if..." with problems that are bothering me today. The phrases are all solutions my physical mind comes up with (no matter how unrealistic) to put my mind at som ease about them.

Today I was feeling bad about having no money until the next couple of days until I get paid, so I wrote down "Wouldn't it be nice if my dad were to just randomly give me $200?"

I use this in conjuction with Bashars definition of abundance: the ability to do what you need to do when you need to do it. I needed money to go to gymnastics, and my imagination really assits the process, "Wouldn't be nice if my friend shouted me at gym today?"

I get out a second word document, and put all of my inspired action onto that one. It goes from thoughts floating around of document 1 to proper solutions on document 2. I jazzed up the second document with cool font and pictures, sort of showing the power of my imagination and the bliss my higher mind gives me when in alignment with the physical!

Note, I don't try and get inspired action. It literally just pops into my head, about certain subjects. Some subjects I have absolutely zero imagination about whilst others I can transform and mould vibration easier. Personally this is my great alternative to focus blocks (I never 'got' focus blocks, I feel guilty saying that).

Afterwards I went through a Rampage of Appreciation and had inspired action to get busy and have fun with my life, as I always find a way to do!