How should I price my design services?

There can be two approaches; 1.
You can charge your client on hourly basis, in this case you won't need to worry about how many times you are client is asking to redo, tweak the design.
You can set your hourly rates and turn around time for logos, banners etc.
2.
You can also go for fixed price model, provide your client with a fixed price after reviewing the requirements.
You can offer number of mocks, updates, tweaks in the quote to safe yourself from too many redo requests.
This model will be good for managing multiple projects.
Hope it helps.

Simply Each project is different from the other and each client is different from the other, the experience can know the right price

Depends on the project, the client and on the hours you'll spend.
Sometimes if it's a quick project that won't take much time a one off charge works out better then an hourly rate.
However if you know you'll be spending a while on something hourly rates can bring you the most money.
If you know a client is willing to pay a lot to get something done, in those cases you can aim for something a little higher.
When I started I didn't want to hit too high and did £12 an hour.
As I become better and more confident in my work I started going as high as £30 an hour.
It's probably still low but it all depends on who and what the work is.
You don't want to cause upset and lose customers so you need to play it by ear.
Ask around your design community and see what your friends price, sometimes that can help you pick a figure.
Good luck!

I would price my design services by planning and organizing people, infrastructure, communication and material components of a service in order to improve its quality and the interaction between the service provider and its customers.