For thieves, not being the martial wonders that fighters are (at toe to toe anyway), we must rely on other methods. There is an old saying that goes "Walk quietly and carry a big stick" but thieves dont learn the use of very big weapons so the saying should go, more properly for thieves, "Walk quietly but be prepared to stomp around and swagger a bit."

Not as smooth sounding I know, but very true. If people have never seen you in battle, but you give off the impression that you are not to be messed with and anyone stupid enough to attack you will die sooner or later, horribly, you are using proper intimidation techniques.

It would be dumb for you to go around with a chip on your shoulder, getting in peoples faces and causing that sort of trouble (unless you can back it up). But you should work hard on giving out the vibes that you are a dangerous character.

Clothing:

One of the most important choices in intimidation is your choice of clothing. No one will respect a thief in a pink cloak wearing a powder blue flop hat, no matter how good you are. (People may just be scared of you if you dress like that but, I wouldnt suggest it.)

The obvious choice for a thief is black, it blends in well with shadows, it looks thiefly and its readily available. Any thief worth his weight in salt should at least be wearing dark colors.

Though not intimidating alone, dark clothes add that look of danger and stealthiness.

Weapon size isn't much of a factor when using this technique. I believe that those who seem to not have weapons are the most intimidating. Conceal a good weapon somewhere in your cloak and try to look like you are carrying nothing. At the least when you draw it you will have the element of surprise.

Sometimes intimidation isn't enough. When that time comes its good to be able to back up your words with force. As a member of the Thievesguild you can purchase extra "muscle". These "Thugs" can only be so helpful though as they are easily beaten off and the purchase of them is frowned on by most citizens. If you bought the power of these fellows a few times soon everyone would probably dislike you and consider you a bully. "Thugs" are acceptable muscle occasionally.
Instead, you will want to make real friends, friends you can trust, and travel with when the need arises. Preferably they are loyal to you for one reason or another and can fight and will fight for your reasons. These are the best kind of friends to have.
With these as your "Force" you should be able to carry out your threats or whatever you believe the force will be good for.

If you dont have a loyal force to back you up, you can consider paying for one. This is a risky venture as you dont know that you can actually trust the ones you hire to fulfill their orders. But in a pinch or as extra "muscle" buying friends is great.

Not only can you purchase the services of mercenaries, you can pay off those in high places who can be damaging to your target. You can also pay off jailers, town guards, lawyers, people who can frame your target and so on and so on and so on!
All that you need is a good supply of cash and the knowledge of who is corrupt and/or bribeable.
Be sure to not over look exchanging favors instead of crowns and never underestimate the power of a fake blackmail. You can slander someone really badly in our kingdom with a few well paid "witnesses".
An old trick to get at someone is to frame yourself! When you do this, do it carefully. Leave holes in the story that will be obvious to someone thinking about the crime, you will of course deny it and will be able to answer honestly under a truth spell. Make sure you make it look like your target framed you and either did the crime themselves or made it up. You then charge them for what is called Slander (The act of trying to ruin someones name with lies) and you ask that they be punished, pick something that isnt to hard on them, like jail time that way you look like the good guy!
Do this smoothly or you'll have a big mess on your hands, and it will be either your own blood or the dirt from the jailroom haystacks.

But no thief will ever be as equipped to deal with problems or jobs as the STEALTHY thief. I have practiced the art of stealth at GOA and have read from many different sources on how to accomplish silent movement, "invisibility", hiding, and using an opponents mind against them.

Some of THE best books were available at Barnes and Nobles in their martial arts section.
Find the section and look at all the books on Ninjutsu (The martial art of the ninja). They almost ALL have sections on stealth, silent movement and ambush. There are also several highly useful books I have found regarding using inner magic and outer magic to become stealthy and "invisible". The passages and so on from the books that I own are below.
Being stealthy is a true SKILL and "invisibility" is not impossible if you understand the basics of sight and other senses. Below is a short collection of things that will help you in many different ways. Eastern philosophy holds MANY clues on how to become a stealthy warrior. Well without further ado, heres a few things that will help you in the art of "invisibility".

Always move with stealth like a watchful fox. Make your footsteps silent and let every step carry respect for the ground that supports you.
Think your footsteps light; A warrior does not clomp about like a drunken elephant. Lightening your step will not only save your shoe leather and enable you to walk without disturbing the delicate field of sonic vibrations surrounding you, but will also help keep your energy and consequently your thoughts and emotions lighter and more level.

Next time youre walking down the street with nothing more pressing on your mind, visualize a cup of tea in your tantien (the psychic center below your navel). Your challenge is to retain all the tea in the cup as you walk, without spilling a single drop.
or
Imagine the ground is covered in gold leaf and your challenge is to walk without tearing it.

With practice you'll be able to appear suddenly, as if by magic, without any sonic vibrational warning.
You enter each situation without telegraphing your arrival and leave without a trace. This ability is linked with the skill to make yourself invisible and with the ability to contain your energy. The lightness of your steps relates directly to the lightness of your energy, emotions and thoughts, and vice versa, though lightness does not mean airiness. Every footstep must make intelligent contact with the ground. The soles of your feet are important receptors which collect information from the ground.

Everything that happens on this planet, from a small fly landing on your nose to a nuclear device being detonated, cause vibrations, both in the air and in the ground. These vibrations carry complex information which is assimilated automatically by your unconcious, instinctual mind. This is how animals can sense an impending earthquake-and you have this same ability.
This quality of listening through your feet is a major component of your potential psychic power. To develop it try this walking meditation:

Imagine youve gone out and bought two state of the art microphones and had one installed in the sole of each foot. As you walk along imagine these microphones are picking up the information carried in the vibrations in the ground. When your thoughts are fairly still you may recieve information translated into pictures which will help guide you on your way.

This is part of the process of sensitizing your body as a vibration receptor, as in buying a bigger satelite dish, in order to recieve a broader spectrum of information and hence increase your powers of intuition.