"The word is more important than the letter; the sentence is more important than the word and the page is more important than the sentence." I forget the wise person who said that, but I believe that it's absolutely true with handwriting and lettering in general.
We instictively seem to like symmetry and the most important first factor when assessing a page of handwriting , is consistency of slope (or the lack of it, in the case of upright writing). Even indifferent writing will look much more attractive if the slope is even and regular. If you feel that you may have this problem, try the following -
Write 2 or 3 pages in your normal hand at normal speed. View them at a distance which takes in the whole page and assess the most common slope for your handwriting. This becomes the slope at which you should always practice this hand.
Having established the angle, carefully draw up a page in black ink (a fibre tip is perfect for this) of horizontal and angled lines at your slope. Space the angled lines no more than 20mm apart. I use a clip board with the base sheet fixed permanently in position. For practice, 80gsm copier paper is ideal as it has sufficient see-through. A few sessions of writing with slope awareness like this, can produce remarkable results.
All the other important factors can then be addressed - letter structure, spacing etc. Even if these factors are OK, handwriting will never look good if the slope is all over the place. If a consistent slope is used from the beginning, everything else will fall into place, with practice. Idiosyncracy in handwriting style comes later, when sufficient control over the basics has been achieved, and I believe that starting with slope consistency is the most important first step.