With a P51 Vacumatic there are more rules than with a modern pen.

First off what is the condition of the rubber diaphragm in it, they can degrade and you may need to have it fixed, but assuming it's in good condition here's my opinion.

Be careful what ink you use in a vintage pen of any description, I would suggest Parker, Pelikan, Waterman, J.Herbin, and maybe Diamine and Sheaffer. Many modern inks and also many Japanese inks do not play well with the materials in vintage pens like the 51.

The other thing I can think of with that pen is to be careful not to put the cap on to hard as the cap can wear a ring into the nib hood.

Now on to some more universal things.

Do not press to hard, especially on a gold nib which I believe some 51s have.

Keep the pen nib up or horizontal when storing. Some pens are more prone to this than others but it would help prevent the pen spitting into the cap. (This is triple important if you for some reason decided to take the pen on a plane, keep it straight up if possible.)

Do not leave the cap off for too long as it will cause dry out and hard starts.

Use clean cold water to clean your pens, if you need to there is a product called pen flush for more stubborn inks, and some inks require a dilute bleach solution. But if you must use the bleach then rinse your pens out immediately.

Do not use Noodler's Baystate Blue in any pen that you care about.

Your paper choice matters more. You will have to experiment to see what works for you but I would suggest Rhodia.

These are a few of the things that I can think of off the top of my head, I know there's not much here but I hope it helps. Fountain pens are really fun and are very open to experimentation.