One common problem that I come across is the lack of thought into equipping your practice space with all of the tools you need.
I have compiled a list of must haves for your practice room in no particular order.
01. Suitable chair - Unless you are practicing for playing in a band or a situation where you stand. Then the number one item you can have is a chair that is suitable. Typically it should be flat, not have arms on it (obvious?) and from my experience the part you sit on should be in line with the back of your knee when standing infront of it.
02. Music Stand - Much easier than propping your music up or putting it down on a bed or the floor. Invest in a music stand!
03. Notebook & Pencils - These come in handy for writing any changes you have to make in fingering or for marking out sections. Also for any breakthroughs you have and keeping a track of what you are practicing. Put it in your notebook.
04. Metronome - Keeps you right with timing and also when used correctly will enhance the speed and thoroughness of which you learn new material.
05. Mirror - A great tool in checking yourself for tension and bad habits.
06. Clock/Timer - Keeps track of how long you have worked on certain things and lets you know when to take breaks which will make your practice more efficient.
07. Audio Recorder - As much as we don't like to listen to ourselves when it's not perfect. Recording sections of your practice will enable you to hear it as it really is and highlight what needs more work. We don't always sound like we think in our head when we are playing.
08. Lighting - To prevent eye strain or reduced energy levels. Proper lighting will aid your practice.
09. Quiet & Privacy - If you are practicing properly then you will sometimes be playing the same bar over and over and over....without privacy you are going to drive anyone who can hear a little crazy. Turn off the phone/computer and be distraction free. When it's time to practice it is time to practice.
10. Tuner - Please have the tools to tune up your instrument before you play. This could be a tuner, pitch fork, piano...as long as you are playing in tune.