What are some suggestions for keeping the Tefiliin Retzuos (straps) black? How can I prevent the Retzuos from needing to be painted and/or replaced?
Always keep one of those handy tefillin paint pens in your outer tefillin bag. Check them once a month for need of paint on erev Shabbos and repaint accordingly.
The only way to prevent retzu'os from ever needing to be repainted or replaced is to not wear your tefillin. Even then, due to lack of contact with your natural skin lipids they would become hard and brittle.
I find that the specific leather used in the Retzuos seems to matter. The retzous that my dad got me with my tefillin at my bar mitzvah were very soft and supple, but the paint cracked and it was difficult to maintain.
The replacement pair I got have been pretty much maintenence free for the last 15 years or so.
I wish I knew how to describe the differences in real terms, but clearly there is a major difference.
If the all black retzuos have a kosher shade of black on the insides/sides and back then when the glossy black paint wears off (They put a layer of gloss over the painted top) a matte shade of black will be revealed. This is also a kosher shade of black. Be sure to choose a pair of retzuos in which the underside/sides/inside are black as if they aren't then there's no difference between these and the regular retzuos and the surface would require painting when the black paint wears off.
Leather that is too soft, while very comfortable, will stretch. Leather naturally stretches and paint doesn't so it will crack. Quality retzuos are 14-15mm wide. If they stretch to less than of 10/11mm wide they are bedieved and less than 9mm may e pasul. They commonly stretch at the point where they are tightened on the upper arm.