Separation of action and reaction

The greater the distance between a user’s action and the system’s reaction, the more you need to emphasise the relationship between the action and the reaction.

Distance can be measured in space, or in time.

Play tricks, don't wait for a full-house

Deciding when your design is mature enough to put in front of a client is always difficult.  Unfortunately, there is a natural tendency to err on the side of holding off for longer than we should.
All other things being equal, you should expose your design earlier rather than later - in fact, earlier than you feel comfortable with.
Even if you have problems with aspects of the design, it is better to bring your stakeholders over to your side of the problem sooner.
To use an analogy from card games; play your design out in small "tricks" don't wait for a "full house", lest you be left with a hand full of great - but now useless - cards when the project moves on without you.

Butcher's paper is your friend

Lay it around your desk as you work.  Take notes, draw little diagrams.  Draw screen mock-ups.  Pin them on the wall.  They won't get lost like pages in your notebook if they are visible in your workplace.  Additionally, your process and progress will be visible to other teams.

Find a pen that you love drawing and writing with

A beautiful, well weighted pen that makes your sketches and scratchings look like the kind of thing found in the notepads of architects (rather than on the walls of a Mexican prison) is an inspiring tool to have in your kit-bag.

In addition to drawing envious glances from clients and designers alike, it will increase your desire to sketch and take notes.

Getting the pen moving early and freely circumvents one of the biggest barriers to completing a design... starting.

Fluffy clouds are your friends

In early iterations of the design, it is better to get a breadth-pass than to get bogged down in the depth of one particular detail.  Use fluffy clouds - unashamedly - to communicate that you simply don't know what will appear in that part of the design yet.  Better this approach than detailing an ill-thought out guess which might cause the kernel of a good idea to be tossed into the chaff-pile by a stakeholder with a critical eye for detail.

In times of trouble, ask "What is the user problem that we are trying to solve?"

Emphasis here on "...PROBLEM WE ARE TRYING TO SOLVE".  We easily get caught up in fitting buttons and fields on screens etc.  But often - even after requirements have been detailed - we loose sight of the fact that we are trying to solve business and human problems with the functions and information that we are designing.  Asking this question helps reorient the project back toward what is important. When elevated to a mantra, other teams on the project start asking the question and you know an attitude change has been affected.

Functionality is the enemy of usability

When you are handed a functional specification, it is easy to take it on face value, and then work hard to make that functionality as usable as possible. If you are really focussed on usability, then your job starts with critiquing the functionality itself.

Your design isn't finished until you can't take anything else away

...without sacrificing user or business needs

Similarity implies relationship

Visual designers tell us to use as few colours as possible (especially if we are novice designers). That may mean we apply the same colour to two (or more) items on a screen. Be careful, though, that the similar colour (or other treatment) does not imply a relationship you do not intend.

The level of detail in your mock-ups should reflect the level of confidence in your design

Mock-ups are prepared as a way of exploring a particular design problem, and in order to communicate your thoughts on a solution to important stakeholders (i.e. users, clients or developers).
In the early stages of design, mock-ups should communicate broad concepts and groupings of information or functions.  Keeping the details scant at these early stages allows you to progress through iterations quickly and keeps your stakeholders from throwing the baby out with the bath-water by quibbling over colours and button placement.
As you progress towards a final design, mock-ups should be rendered in increasingly higher fidelity so that the full detail of your approach is exposed.