Observing the many students who begin training in brazilian jiu-jitsu every year at the academy, there are some very common pitfalls that the beginning student must be wary of falling into.
It is not only whitebelt students who may make these training mistakes: one must guard against these negative habits throughout the course of your bjj. 

1) Burn Out

Many who begin training in bjj quickly fall in love with the sport and totally immerse themselves in the jiu-jitsu lifestyle. They buy several new kimonos, watch all of the Youtube videos and embark on an intense physical conditioning program trying to catch up to the more experienced students.
Some may even train every day or multiple times per day!
However, the saturation of all of this in the mind can lead to mental burn out. And in the worst cases, the student may abandon their training altogether!

To avoid this one must find a balance between training and the other aspects of a healthy life.
Remind yourself that any one can start quickly, but it is even, consistent participation over a sustained period that results in a true jiu-jitsu lifestyle.
One of the students at my Gracie Barra Academy has a great quote that applies to training burnout:

“Consistent effort is more important than occasional times of sprinted effort.” 
GB student Doug Vermeeren


2) Unrealistic Expectations
Your progress in training can be frustratingly filled with stops and starts, hiccups and obstacles. Improvement is not always a steady progression and a frequent complaint of practitioners is that their training has become stale.

While all of us wish to get better at the fastest possible rate, we must accept that non-linear progression is the nature of anything that is challenging.

In many cases of frustration the student has placed a time frame on their expected progress. They feel that they should have achieved a certain belt in a certain amount of time and anything less is cause for disappointment.
They may see another student who started around the same time who appears to be progressing faster. Maybe they heard that a bjj phenom achieved their black belt in a relatively short period of time.

Comparing ones own progress unfavourably to another who may have very different natural talent, time to train and commitment can lead to the trap of feeling your own jiu-jitsu is lacking.

Remind yourself periodically that each person’s bjj journey is different and instead view your own study of bjj as a marathon, not a sprint to the next belt.

3) Wrong Mind set in Rolling
Bjj tends to attract competitive types of personalities. It is very natural to want to show your best and apply your maximum effort each time you visit the academy.
There is another side to the competitive mentality that could end of costing you something more important than a victory in any single roll: your longer term development in jiu-jitsu.

Those students who possess natural athletic attributes will be tempted to use them at the expense of using the proper technique just to get a win. This comes at the expense of using proper technique.
The problem with this is that it has a limited period of success before those will over take you using superior technique. Many smaller students are known to be the most technical in the academy because they have always had to rely on technique in the absence of size and explosiveness.

In my first year of judo my instructor took me aside and said very diplomatically that he could see that I was strong (I was a serious weight trainer at the time) but not to use too much power in my judo.
Her said to try to develop the technique by setting aside the power for now. When I developed the technique and then added the power, the combination would make me a very difficult opponent.

Remind yourself that the purpose of rolling in the academy is to DEVELOP your technique (try to use the techniques your professor shows you), not to prove how difficult it is to tap you.