1° Always use a bow stringer to string or unstring your bow.
Using the bow stringer is important, even if you can do without.
Each time you string a bow without a bow stringer, you bring inevitable torsions to the limbs, causing them to twist more or less depending on the technique you use.
It is a known fact for recurve bows but applies less to longbows, because of the narrowness of their limbs.
However, I recommend you always use it, if only to spare your back.
2° Quickly control the string before each use of your bow.
Grease it regularly, especially where frictions are more significant.
Should you have any doubt concerning its condition, change it.
Check that the loops sit correctly in the grooves of the bow tips.
If you are a regular shooter, change your bowstring every 6 months if it is made of Dacron and every year if it is made of Fastflight, Astro flight, etc.,...
3° Regularly check the condition of your arrow nocks to avoid breaking upon release.
It may cause cracks, delamination or be lethal to your bow.
4° Do not use a bowstring with less strands than recommended by the ATA standards for a given draw weight.
It may be tempting to in order to gain a few feet per second, but it will be at the cost of your bow lifespan and comfort during shooting.
5° Do not use arrows with less than 8 grains per pound.
Here as well, spare your bow. The more you will chose to shoot arrows with 8 grains per pound or more, the more lifespan your bow will have and the more pleasant it will be to shoot.
Generally, look after your equipment and you will never have any problem with it.
Do never forget that, even if shooting a bow is a playful enjoyment, this object is a weapon.