The best general advice is to slow down while writing. Here are 25 other helpful suggestions:
The key to practice is repetition. Try to do a whole line of a single letter, making each one look as good as the last. Once you have consistency down, try increasing speed. These are like practicing scales on a musical instrument, or vocabulary drills on a foreign language: boring, but necessary if you want to see significant improvement.
a: consider shaping it like the a to the left, then start on the part that goes above the x-height and make a full counter.
b: start on the top of the ascender and make a full counter.
c: should be as round as e and o.
d: start on the top of the ascender (not the circle) and make a full counter.
e: make this a loop like a cursive l.
f: consider going below the baseline with downstroke.
g: full counter, loopy tail.
h: should look very similar to b.
i: tiny circle for dot.
j: nice loopy tail or hook on descender, tiny circle for dot.
k: don't make angled arm and leg straight lines.
l: this should set the slant for all other letters.
m: nice rounded arches, consider a small hook on front and back.
n: nice rounded arches, consider a small hook on front and back.
o: one of the hardest-- a good o with a counter and perhaps a nice flourish is hardest to make consistently, so practice!
p: start at bottom of descender and write up to loop.
q: should look like mirror of p. You may need to start with loop, and give descender a nice flourish. make it similar to, but distinguishable from, your g.
r: nice rounded arch like m and n.
s: make lower curve in s much bigger than top.
t: this should set crossbar for all other letters.
u: should look like upside down n.
v: one place where a sharp angle is good. Consider a curved upstroke.
w: consider rounded at baseline, or curved upstroke at least.
x: consider curved second stroke.
y: rounded at baseline, nice curved tail that matches g.
z: consider curved on horizontal lines.
Capital letters: Add flourishes whenever possible.