There are many layers to truthfulness. Some are quiet obvious, stating facts as they are: not telling your boss you have a dentist appointment when really you have an interview elsewhere. Truthfulness also means stating the whole truth: not telling your boss you're not coming to work because your voice is gone and failing to mention it's because you were in a real loud party last night. Exaggeration is another face of untruthfulness and we all do it sometimes to make a story more interesting or sound a little bit cooler! Gossip, adding/removing changing facts and manipulation are all gross practices stopping us from becoming truthful.
Digging deeper into truthfulness, it can be something as subtle as being authentic and sincere. When you convey a concept, idea or belief that you do not necessarily buy into, that you had not necessarily experienced. You are allowing others to believe something which is not true. A good example here may be false compliments (VERY common in the Arabic culture). Another good example is suggesting something to others that you do not practice yourself.
I strongly believe that self study is a huge part of truthfulness. When you understand your thought pattern, desires, fears, strengths, weaknesses and intentions then you are more likely to commit to the truth. This can be obvious in a number of ways:
Firstly, when you are aware of who you are you will follow your life's destiny, you will not waste your life in a job that you cannot grow in, or a relationship that doesn't serve you or a life that you don't belong to. Simply put you'll quit lying to yourself about who you wish you were, or knew others wished you were and allow yourself to completely be you.
Secondly, when you understand yourself it becomes easier to tell yourself the truth. It's then easier to "name it" as my yoga teacher calls it. You're acting violently towards someone and you know it's because there's jealousy there. You're putting on an act in front of someone because you're attracted to them. You're avoiding spending time with a certain group because you're intimidated and scared of not being accepted. Once you are brave enough to "name it" life becomes so much easier as you stop trying to create excuses and accept things as they are.
Thirdly, when you understand yourself, you understand your intentions. Intentions are quiet tricky because you may be saying something which is true but if your intention behind it is impure then it stops being true. Lets say for example you are out with a friend shopping and she takes AGES to pick out the perfect pair of shoes. When she asks if the 100th pair looks good you enthusiastically say "YES! It looks GREAT!". While the shoes may well look great, you only said it because you'd rather be home tucked in bed. Your intentions were impure which turns your truth to untruth.
A lot of us find it scary to look truth in the eye, but trust you me, the monster will only become bigger if you leave it. Look it in the eye and it might go away. Go on, look inside, find your truth, name it and enjoy the treasures you'll find within.