First of all cylinders can't stand on their own, a cylinder boot helps it to stand up and protects the bottom from damage. This is a definite must for your dive cylinder. Do take the boot off from time to time and see if there is any damage to the cylinder.
Basic Cylinder Care
Handling - First of handle them with care, don't drop and bang them. You should remember, a filled cylinder is a bit like a small bomb (you might have heard a tyre burst (that is only 2 to 3 bar), imagine 232 bar exploding (a full cylinder exploding in your car will rip open your car like a can of sardines - so do handle them carefully).
Moisture - The single biggest damage to a cylinder is moisture. If moisture gets into the cylinder or you suspect water ingress into the cylinder, get it checked out and tested immediately. Moisture under pressure will have an accelerating effect inside the cylinder and it can corrode very quickly. The easiest way to avoid this is by getting your gas fills from a reputable dive centre source (the double filter usually) and always keep the cylinder under pressure (try to keep it above 20bar all the time).
Storing - Keep the cylinder in a cool and dry area. Also try not to store it full pressure for long periods of time. If you need to lower the cylinder pressure, release the gas slowly. A fast gas release (apart from the noise it makes), will cool the cylinder down quickly and moisture can form on it (and risk getting into the cylinder).
Tests - By law you must get your cylinders inspected and tested in regular intervals. A visual inspection every 2.5 years and a full hydrostatic test (where they strip down the cylinder, check it and then test how much it expands under pressure) every 5 years. If the cylinder is out of test, the filling stations will not fill it. The reason for the test is to check for corrosion (visual), but as you can see from the photo (courtesy of Massdiving.com), the walls of a cylinder become thinner over time from the constant pressurisation.
Cylinder Neck Markings - Therefore it is advisable to understand what the markings on the cylinder actually mean.
In the UK, the approved Government Agency rating should be EN144/BS (in the US you will see DOT/CTC).
The metal type tells the tester what the "expansion" tolerances are during the test - 3A stands for Carbon Steel.
Working Pressure - is the maximum normal operating pressure for the cylinder, for testing the tester will over pressurise the cylinder to a set fixed percentage.
Serial Number - a unique number issued by the
Manufacturer (in this example Faber, but it could also read luxfer, PST in the UK - being the most popular manufacturers).
Hydrostatic Test Date is 6/2012 on this example, meaning the next visual inspection is due at the latest in 12/2014 (with a little diamond in between to show it is from an approved test station).
The PLUS symbol means a 10% Overfill is allowed.
General advise - If you buy a second hand cylinder, always ask for the seller to supply a valid testing certificate with the cylinder. Any cylinder more than 12 months from its last test (unless you know where the cylinder has been), we wouldn't touch it.